This week, as Brian Doherty noted on Tuesday, the Supreme Court ruled that deploying a drug-sniffing dog at the doorstep of a home qualifies as a search under the Fourth Amendment, meaning it generally requires a warrant. The case involved a Florida marijuana grower, Joelis Jardines, whose house was searched after a police dog alerted to his front door, thereby providing the basis for a warrant. Since the Court already had ruled (in Florida v. Harris, decided last month) that such an alert provides probable cause for a search, its decision in Florida v. Jardines is especially important. If it had ruled that police and their drug-detecting canines may visit people's homes at will, any cop with a dog would have been able to obtain a warrant to search anyone's house simply by asserting that the animal had signaled the presence of contraband.

The majority in Jardines is an interesting mix: Antonin Scalia and Clarence Thomas, conventionally identified as the most right-wing members of the Court, joined by Ruth Bader Ginsburg, Elena Kagan, and Sonia Sotomayor, three members of the Court's "liberal" wing. All five agreed that bringing a police dog to a home with the aim of discovering illegal drugs amounts to trespassing. As Scalia puts it in the majority opinion:

The officers were gathering information in an area belonging to Jardines and immediately surrounding his house—in the curtilage of the house, which we have held enjoys protection as part of the home itself. And they gathered that information by physically entering and occupying the area to engage in conduct not explicitly or implicitly permitted by the homeowner.

Because "the officers learned what they learned only by physically intruding on Jardines' property to gather evidence," Scalia explains, it is not necessary to decide whether their investigation violated Jardines' "reasonable expectation of privacy," the standard established in Katz v. United States, a 1967 case involving eavesdropping by FBI agents who bugged a telephone booth. Kagan, joined by Ginsburg and Sotomayor, goes further in a concurring opinion, saying a drug-detecting dog at the dooorstep also meets the Katz criterion.

In U.S. v. Jones, a 2012 decision involving GPS surveillance, Scalia likewise emphasized the physical intrusion involved in attaching a tracking device to the suspect's car, while a concurring opinion by Samuel Alito (joined by Kagan and Ginsburg) preferred the reasonable-expectation-of-privacy approach. Although the trespass test and the expectation test led to the same outcome in these two cases, they have different implications in other situations. By using cellphone geolocation, surveillance cameras, or drones, police can obtain evidence similar to the tracking data at issue in Jones without touching the target's property. Similarly, police can subject a residence to a canine smell test without trespassing as long as they remain on public property, as opposed to someone's porch or the private path to his door. So if you live in a townhouse or apartment that abuts a public sidewalk, Scalia's analysis in Jardines does not help you. Likewise if you live in an apartment building whose landlord lets cops roam the halls with their dogs.

As Steve Chapman noted in a recent column, Scalia could have provided more protection for privacy by applying the logic of his opinion in Kyllo v. United States, a 2001 decision that said using thermal imaging to identify the homes of marijuana growers qualifies as a search even though it does not involve trespassing on anyone's property. Kagan suggests as much in her concurrence, quoting this seemingly relevant sentence from Scalia's majority opinion in Kyllo: "Where, as here, the Government uses a device that is not in general public use to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a 'search' and is presumptively unreasonable without a warrant."

One reason Scalia chose not to put a drug-detecting canine in the same category as a thermal imager may be that doing so would sit uneasily with decisions in which the Court has held that using a police dog to sniff out drugs in a car or a suitcase does not constitute a search. As I argue in the March issue of Reason, those rulings are based on an exaggerated view of canine capabilities, according to which an alert by a "well-trained narcotics detection dog" signals nothing more or less than the presence of contraband. In reality a police dog's alert may indicate nothing more than its handler's suspicions, magically transforming a cop's hunch into probable cause. The upshot, given all the pretexts that police can use to pull people over, is that a cop with a dog can search any vehicle he wants. Although the justices missed an opportunity to restrict that power in Harris, at least they did not extend it to homes in Jardines.