In the case of Illinois v. Caballes, the U.S. Supreme Court ruled in 2005 that police do not need reasonable suspicion to use drug detection dogs during a lawful traffic stop. This decision came after Roy I. Caballes was stopped for speeding and subsequently arrested for marijuana trafficking when a drug-sniffing dog alerted officers to illegal substances in his trunk while he was being issued a warning ticket for speeding. The court held that since dog sniffs only identify the presence of illegal items - in which citizens have no legitimate privacy interest - the Fourth Amendment does not apply to their use by law enforcement officials during a legal traffic stop.

In the dissenting opinion for Illinois v. Caballes, Justice Souter argued that using a drug-sniffing dog during a routine traffic stop without any specific reason to suspect drug activity was an unreasonable search under the Fourth Amendment. He contended that this ruling would allow police officers to routinely use dogs in all traffic stops, which could lead to unnecessary invasions of privacy and potential abuse of power by law enforcement officials. Furthermore, he expressed concern about the reliability of drug-sniffing dogs and noted that false positives could result in innocent people being subjected to invasive searches based on unreliable evidence. Overall, Justice Souter believed that allowing such practices would erode civil liberties and undermine constitutional protections against unreasonable searches.