In Rodriquez v. U.S., the U.S. Supreme Court held that officers violated the Fourth Amendment when they extended a traffic stop to wait for a drug-sniffing dog. The lower courts concluded that there was no reasonable suspicion or probable cause to extend the stop. If either of those had been present, the extension would have been lawful The Supreme Court did not deal with that issue. For more detail, see the Courts syllabus for the opinion at the link (“Absent reasonable suspicion, police extension of a traffic stop in order to conduct a dog sniff violates the Constitution’s shield gainst unreasonable seizures”)
Thomas, Kennedy and Alito dissented. This is an issue that the Court had dodged for too long. However, the finally came up with the correct decision.