Illinois v. Caballes
In Illinois v. Caballes, 533 U.S. 27 (2005), a 7-2 U.S. Supreme Court upheld the use of drug-sniffing dogs without probable cause and without a warrant to smell for drugs outside the trunk of a car stopped for speeding. The rationale was that the drugs could only detect unlawful activity in the trunk of the car, and thus there was no violation of the Fourth Amendment.
The Court noted that a lawful seizure "can become unlawful if it is prolonged beyond the time reasonably required to complete that mission," but accepted the state court's determination that the duration of the stop was not extended by the dog sniff. The Court rejected the notion that "the shift in purpose" "from a lawful traffic stop into a drug investigation" was unlawful because it "was not supported by any reasonable suspicion."