In 2005, the United States Supreme Court held in the case of Illinois v. Caballes that when a trained drug dog sniffs a persons automobile, there is no search. As a result, no constitutional violation occurs. This is the case even if the police have absolutely no reason to suspect you may be carrying illegal contraband whatsoever.
The majority reasoned that a dog’s sniffing is not really a “search” because it detects only contraband, and therefore does not compromise the privacy of someone who has nothing to hide. Justices David H. Souter and Ruth Bader Ginsburg both dissented strongly, warning that this decision could lead to “suspicionless and indiscriminate sweeps of cars in parking garages and pedestrians on sidewalks.”
The decision was not shocking, it was merely an extension of an earlier case decided by the Supreme Court in 1982 which held that the use of drug sniffing dogs to search luggage in an airport was in fact not a search either. (United States v. Place).