Today, the Supreme Court issued a decision in Florida v. Jardines
(No. 11-564), holding that "The government's use of trained police dogs to investigate the home and its immediate surroundings is a 'search' within the meaning of the Fourth Amendment."
The police had approached Jardines' home with a drug sniffing dog. While the dog was on the porch, its handler informed the accompanying detective that the dog had alerted for narcotics. On the basis of the positive alert, the detective applied for a warrant, searched the home and found the marijuana plants that later became the basis for drug trafficking charges.
Writing for the majority in a 5-4 decision, Justice Scalia based the Court's holding on property rights grounds: "One virtue of the Fourth Amendment’s property-rights baseline is that it keeps easy cases easy. That the officers learned what they learned only by physically intruding on Jardines’ property to gather evidence is enough to establish that a search occurred."
For more on the opinion, see this SCOTUSblog post