Not to be cited per Circuit Rule 53United States Court of AppealsFor the Seventh Circuit Chicago, Illinois 60604Argued April 19, 2005 Decided May 16, 2005BeforeHon. ILANA DIAMOND ROVNER, Circuit JudgeHon. TERENCE T. EVANS, Circuit JudgeHon. ANN CLAIRE WILLIAMS, Circuit JudgeNo. 04-3314UNITED STATES OF AMERICA, Plaintiff-Appellee, Appeal from the United States District Court for the SouthernDistrict of Illinoisv. No. 02-CR-30060-DRHJOHN M. WYATT, Defendant-Appellant.David R. Herndon, Judge.O R D E RPursuant to a conditional plea agreement, John M. Wyatt pleaded guilty to possession with intent to distribute over 100 kilograms of marijuana, 21 U.S.C. § 841(a)(1), and was sentenced to 262 months§ 4B1.1, as well as the reasonableness of the term imposed. We affirm his conviction and sentence.I. We recount from the testimony at the suppression hearing the early morning traffic stop on January 15, 2002, that led to Wyatt§ 4B1.1. The career offender status was premised on a prior controlled substance offense and a conviction for escape from custody, which the probation officer categorized as a crime of violence. Wyatt objected, arguing that his conviction for a nonviolent escape was not a crime of violence and that, under Blakely and our decision in Booker, the court could not apply § 4B1.1 based on facts not found by a jury beyond a reasonable doubt.At sentencing the district court adopted the probation officer§ 3553(a). II. On appeal Wyatt first challenges the denial of his motion to suppress. He contends that Officer Modrusic intended from the beginning of the traffic stop to search his RV for narcotics and that, because the scope of the stop exceeded its pretextual justification, the stop violated the Fourth Amendment. In addition, Wyatt contends that Modrusic detained him “at least a few seconds, if not a few minutes” beyond the completion of the traffic stop without reasonable suspicion or his consent. In reviewing the district courtÂ’s factual findings for clear error and its legal conclusions de novo, we give special deference to the district court that heard the testimony and had the best opportunity to observe the witnesses at the suppression hearing. United States v. Jackson, 300 F.3d 740, 745 (7th Cir. 2002). The scope and duration of a traffic stop must be reasonably related to its initial purpose. United States v. Brown, 188 F.3d 860, 864 (7th Cir. 1999). Where an officer can articulate grounds that establish reasonable suspicion of criminal activity, he may extend the duration of the traffic stop to investigate that activity. United States v. Walden, 146 F.3d 487, 490 (7th Cir. 1998). Whether reasonable suspicion exists depends on a consideration of the totality of the circumstances known to the officer at that time including his experiences and common sense. Jackson, 300 F.3d at 745-46.As an initial matter, WyattÂ’s argument that Officer ModrusicÂ’s subjective intent vitiated the reasonableness of the search warrants little discussion. Modrusic had probable cause to initiate a traffic stop because Wyatt had undisputedly committed a traffic violation by speeding, and so ModrusicÂ’s subjective intent is irrelevant for purposes of WyattÂ’s Fourth Amendment argument. See, e.g., Whren v. United States, 517 U.S. 806 , 813 (1996); United States v. Moore, 375 F.3d 580, 583 n.1 (7th Cir. 2004); United States v. Robinson, 314 F.3d 905, 907 (7th Cir. 2003); Valance v. Wisel, 110 F.3d 1269, 1275 (7th Cir. 1997). The real issue is whether ModrusicÂ’s request to search the RV after he completed the traffic stop by issuing a warning was a further detention requiring reasonable suspicion or a consensual encounter. See Ohio v. Robinette, 519 U.S. 33, 35 (1996) (recognizing possibility of consensual encounter following completion of traffic stop in holding that officer does not have to inform driver that he is free to go); Moore, 375 F.3d at 584 (consensual encounters between individual and officer do not trigger Fourth Amendment scrutiny). Wyatt provides little reason other than the length of the encounter to disturb the district courtÂ’s conclusion that the post-stop encounter was consensual. Once Modrusic returned WyattÂ’s paperwork with a warning and told him he was “free to go,” his request to search the RV without more did not amount to a new seizure. See United States v. Rivera, 906 F.2d 319, 323 (7th Cir. 1990) (holding that officerÂ’s request to search was part of consensual encounter after officer had issued written warning, returned defendantÂ’s documents and declared him free to go); cf. United States v. Finke, 85 F.3d 1275, 1281 (7th Cir. 1996) (until officer told defendants that he was calling canine unit, court might have upheld encounter as consensual where all documents were returned, defendants were told they were free to go, and officer asked if defendants “had a minute” before requesting permission to search).In any event, even if Officer ModrusicÂ’s questioning amounted to a seizure, he had reasonable suspicion to detain Wyatt further in order to investigate whether Wyatt was transporting narcotics. Modrusic listed the following factors as contributing to his reasonable suspicion: (1) WyattÂ’s nervousness, (2) the unusual height of the bed visible through the RV window, (3) WyattÂ’s exchange of a rental car with a rental RV for his return trip from Phoenix, (4) his driving alone in an RV back home to a colder climate, and (5) his prior criminal history. In the face of this list, WyattÂ’s attempts to explain away his shaking by citing the cold weather and characterizing the other grounds as “specious,” do little to deflect the specter of reasonable suspicion. This is because Wyatt never confronts the most suspicious factors Modrusic cited: the unusual height of the bed when considered along with the one-way rental of a type of vehicle favored for transporting large quantities of drugs. See United States v. Salzano, 158 F.3d 1107, 1112-12 (10th Cir. 1998) (reasonable suspicion when considering both use of tractor trailer, which is often used to transport large amounts of drugs, and an additional suspicious element of vehicle); see also United States v. Patterson, 65 F.3d 68, 71 (7th Cir. 1995) (recognizing modification to tailgate interior—missing screws—a factor supporting probable cause); United States v. Seals, 987 F.2d 1102, 1107 (5th Cir. 1993) (considering modification to rear seat a factor supporting probable cause). Although there may be an innocent explanation for each individual factor Modrusic identified, when considered together against the background of a narcotic officerÂ’s experience, the factors rise to reasonable suspicion of drug activity. See Finke, 85 F.3d at 1280 (although possible innocent explanation for each factor, reasonable suspicion where car was a rental, travel plans to California brief and made straight through without stopping, and defendants nervous and evasive); see also Jackson, 300 F.3d at 746 (knowledge of criminal record can contribute to totality of circumstances); Brown, 188 F.3d at 865 (nervousness can be considered as one of several factors). Given the existence of reasonable suspicion, we need not address WyattÂ’s challenge to the district courtÂ’s finding that he consented to the dog sniff of the RV exterior. A canine sniff of the exterior of a vehicle that reveals no information other than the location of narcotics does not implicate any separate Fourth Amendment concerns; Wyatt need only be lawfully detained. See Illinois v. Caballes, 125 S. Ct. 834, 838 (2005); United States v. Gregory, 302 F.3d 805, 810 (8th Cir. 2002); United States v. Dortch, 199 F.3d 193, 197 (5th Cir. 1999). Regardless, WyattÂ’s reliance on the videotape to establish that he did not consent to the canine sniff cannot overcome the deference that we pay to a district courtÂ’s factual findings. According to Wyatt, the videotape never shows “any sign” that Wyatt “talked or motioned with his head to give consent.” Objectively, however, the tape is of little value on this issue because WyattÂ’s face is so overexposed from the light of Captain McKinneyÂ’s flashlight that one could not see what he said to the officers and the tape did not record the conversation. And WyattÂ’s rationale that he would not have consented to the dog sniff if he did not consent to a search is merely an alternative theory and does not undercut the courtÂ’s reasoning that Wyatt did not believe the dog would find the narcotics based on the cold weather and their location. See Robinson, 314 F.3d at 907 (disturbing district courtÂ’s credibility finding only where court credited exceedingly improbable testimony). There is no reason to disturb the district courtÂ’s finding that Wyatt consented to the canine sniff, and so it did not violate the Fourth Amendment. See Moore, 375 F.3d at 584. In a final effort to suppress the seized marijuana, Wyatt asserts that, even if the canine sniff did not violate the Fourth Amendment, the dogÂ’s positive alerts to drugs did not provide probable cause to search the RV. Probable cause was lacking, Wyatt contends, because the government failed to establish that the dog, Seer, was sufficiently trained and had an accurate track record in detecting narcotics. To shore up his claim that Seer was not reliable, Wyatt points to the dogÂ’s alerting to sites in the RV where drugs were not found (the driverÂ’s door and the rear passenger corner). WyattÂ’s reliance on the absence of information in the record about SeerÂ’s accuracy is not enough to overcome the generally accepted principle that a positive alert from a well-trained dog is enough for probable cause. See, e.g., Dortch, 199 F.3d at 197 (dogÂ’s positive reaction to narcotics establishes probable cause); United States v. Thomas, 87 F.3d 909, 912 (7th Cir. 1996) (same). The record establishes that Seer was well-trained; Officer Modrusic testified about SeerÂ’s graduation from a 14-week program and the refresher courses the two take to maintain the dogÂ’s skills. See United States v. Robinson, 390 F.3d 853, 875 (6th Cir. 2004) (it would be inappropriate to require mini-trial on dogÂ’s training and performance before officer can rely on dog alert; it is enough for officer to know dog is certified in drug detection); United States v. Boxley, 373 F.3d 759, 762 (6th Cir. 2004) (once established that dog is certified, evidence as to reliability goes only to credibility, not qualifications); United States v. Sanchez-Pena, 336 F.3d 431, 444 (5th Cir. 2003) (evidence of dogÂ’s certification sufficient proof of training). And WyattÂ’s assumption about SeerÂ’s accuracy based on its alerts to the RV is misplaced; we review probable cause based on information known to the officers at the time of the search, Smith v. Lamz, 321 F.3d 680, 684-85 (7th Cir. 2003), and there is no evidence that Modrusic had any doubts about SeerÂ’s accuracy. Wyatt also does not succeed in his sentencing arguments. Wyatt first makes the familiar argument that escape should not be categorically classified as a crime of violence for purposes of the career offender guideline. We have previously rejected WyattÂ’s contention because of the ever-present potential for escape to become violent. See, e.g., United States v. Howze, 343 F.3d 919, 921-22 (7th Cir. 2003); Bryant, 310 F.3d at 553-54; United States v. Franklin, 302 F.3d 722, 724 (7th Cir. 2002); United States v. Nation, 243 F.3d 467, 472 (8th Cir. 2001); see also United States v. Rosas, 401 F.3d 843, 845 (7th Cir. 2005) (same definition of crime of violence for career offender adjustment and Armed Career Criminal Act). And the Supreme CourtÂ’s recent decision in Shepard v. United States, 125 S.Ct. 1254 (2005), would seem to support the continued categorical consideration of escape as a crime of violence based on its warning against finding facts beyond the judicial record. See Shepard, 125 S. Ct. at 1261.In addition, Wyatt would be hardpressed to establish a Booker error. The district court properly anticipated the Supreme CourtÂ’s ruling in Booker by recognizing that the guidelines may be advisory and imposing an alternative sentence (which happened to be the same as its guidelinesÂ’ sentence). See Booker, 125 S. Ct. at 756-57. Although nonguidelines sentences must still be reviewed for “reasonableness” after Booker, no principled argument can be made that WyattÂ’s sentence was unreasonable. See Booker, 125 S. Ct. at 765-66; see also United States v. Crosby, 397 F.3d 103, 114-16 (2d Cir. 2005) (recognizing reasonableness as a flexible concept and that sentence may be unreasonable regardless of length where judge committed Blakely/Booker error or failed to consider applicable guideline range). Wyatt bases his argument that his sentence was unreasonable on the same losing principle that his escape is not a crime of violence and should not be considered. But the advisory guideline range of 262 to 329 months was properly calculated, as discussed above. See Bryant, 310 F.3d at 553-54. And the district court imposed the lowest sentence within that advisory range. See United States v. Tedder, — F.3d —, 2005 WL 767061, at *8 (7th Cir. Apr. 6, 2005) (recognizing that applicable guideline range available to district court post-Booker without need to justify departure from guidelines).Accordingly, we AFFIRM WyattÂ’s conviction and sentence.