The dissenting opinion was written by Justice David H. Souter, joined by Justice Ruth Bader Ginsburg.
Justice Souter called the use of the dog to determine the presence of marijuana in the car's trunk a "search unauthorized as an incident of the speeding stop and unjustified on any other ground."
"The argument goes, because the sniff can only reveal the presence of items devoid of any legal use, the sniff 'does not implicate legitimate privacy interests' and is not to be treated as a search," Justice Souter said.
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In the majority opinion, Justice Stevens said a dog search conducted during a concededly lawful traffic stop that "reveals no information other than the location of a substance that no individual has any right to possess" did not violate Caballes' Fourth Amendment rights against unlawful search and seizure.
But Justice Souter argued that the "infallible dog" was a "creature of legal fiction."
"Although the Supreme Court of Illinois did not get into the sniffing averages of drug dogs, their supposed infallibility is belied by judicial opinions describing well-trained animals sniffing and alerting with less than perfect accuracy, whether owing to errors by their handlers, the limitations of the dogs themselves or even the pervasive contamination of currency by cocaine," he said.
Justice Ginsburg wrote that the Illinois Supreme Court "correctly apprehended the danger in allowing the police to search for contraband despite the absence of cause to suspect its presence."
"Today's decision, in contrast, clears the way for suspicionless, dog-accompanied drug sweeps of parked cars along sidewalks and in parking lots," she said. "Nor would motorists have constitutional grounds for complaint should police with dogs, stationed at long traffic lights, circle cars waiting for the red signal to turn green."