The Supreme Court heard oral argument yesterday in Florida v. Jardines, a case that examined whether bringing a drug-sniffing dog to the front door of a home looking for drugs was a Fourth Amendment search.

Having attended the oral argument (transcript; audio forthcoming), my sense is that a majority on the Court thinks dog-sniffs at front doors (absent a warrant) go too far. But few of the justices know why. The one who does is Justice Kagan.

What rationale might the Court use to decide the case? Even after United States v. Jones threw open Fourth Amendment doctrine, the instinct for using “reasonable expectation of privacy” analysis is strong. (I’ve joked that many lawyers think the word “privacy” can’t be uttered without the prefix “reasonable expectation of.”) This is where much of the discussion focused, and Justice Breyer seemed the most firmly committed to its use.

But the insufficiency of “reasonable expectation” doctrine for providing a decision rule was apparent when Breyer teed up Jardines’s counsel to knock the case out of the park. There was much discussion of what one reasonably expects at the front door of a home. Neighbors may come up. Trick-or-treaters may come up. Neighbors may come up with their dogs. The police may come to the door for a “knock and talk.” Neighbors, trick-or-treaters, dogs, and police officers may all come up and discover odors coming from the house. What makes the drug-sniffing dog unexpected?, Justice Breyer asked:

Do in fact policemen, like other people, come up and breathe? Yes. Do we expect it? Yes, we expect people to come up and breathe. But do we expect them to do what happened here? And at that point, I get into the question: What happened here?

Joelis Jardines’s counsel could not say what made the dog unexpected.

Perhaps property law draws the line that excludes government agents with drug-sniffing dogs, while allowing other visitors to come to the door. Not so. Justice Alito in particular pressed Jardines’s counsel for any case that had excluded dogs (drug-sniffing or otherwise) from the implied consent one gives to visitors on the walk and at the front door. The argument is unavailing, this idea that Florida’s property law (put into play by the majority holding in Jones, which relied on property rights) solve this case. Florida property law doesn’t exclude dogs from the implied permission it gives to lawful visitors on residential property.

None of this is to say that the government had it easy. Florida’s counsel had uttered just three sentences when Justice Kennedy informed him that the rule from Illinois v. Caballes would not carry the day. In Caballes, the Court found there to be no search at all when government agents walked a drug-sniffing dog around a car stopped for other reasons. (I attacked what I called the “Jacobsen/Caballes corollary” to the Katz decision in the Cato Institute’s brief to the Court, and also in this Jurist commentary.)

It won’t be the rule from Caballes. So what is the rationale that decides this case?

Justice Scalia was on the scent when he reasoned with the government’s counsel about what might be done with binoculars.

“As I understand the law,” he said, “the police are entitled to use binoculars to look into the house if—if the residents leave the blinds open, right?”

Florida’s counsel agreed.

“But if they can’t see clearly enough from a distance, they’re not entitled to go onto the curtilage of the house, inside the gate, and use the binoculars from that vantage point, are they?”

“They’re not, Your Honor.”

“Why isn’t it the same thing with the dog?”

Justice Kagan knows that it is. And she used Justice Scalia’s reasoning in Kyllo v. United States, the precedent that is on all fours with this case.

She recited from Kyllo: “ ‘We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical intrusion into a constitutionally protected area constitutes a search, at least where, as here, the technology in question is not in general public use.’ ” And she asked Florida’s counsel, “[W]hat part of that language does not apply in this case?”

“Franky’s nose is not technology,” he replied, referring to the dog. “It’s—he’s using—he’s availing himself of God-given senses in the way that dogs have helped mankind for centuries.”

The existence of dogs in human society for centuries might help the government if dogs had been used for drug-detection all this time. And then only if the question was what it is reasonable to expect.

What matters is that a drug-sniffing dog is indeed a form of sense-enhancing technology. Selected for its strong sense of smell, and trained to convey when particular odors are present, a drug-sniffing dog makes perceptible to law enforcement what is otherwise imperceptible.

And that is the very definition of searching. At least as Black’s Law Dictionary has it: “‘Search’ consists of looking for or seeking out that which is otherwise concealed from view.”

Police officers use dogs to search for drugs and other materials in which they are interested but which they cannot see by themselves. A drug-sniffing dog is a cuddly chromatograph.

And just now, quietly, you have seen at work the rationale that the Supreme Court should use to decide Florida v. Jardines. Was it a search to bring a drug-sniffing dog to the front door of a house? The Court should apply the plain meaning of the word “search” to the facts of the case that has come before it. There’s no need for doctrine at all.