In 2007, Judge Richard Posner found it "untenable" that attaching a tracking device to a car is a seizure. But the Supreme Court struck down warrantless attachment of a GPS device to a car on that basis in 2012. Putting a tracking device on a car makes use of it without the owner's permission, and it deprives the owner of the right to exclude others from the car.

The weird world of data requires us to recognize seizures when government agents take any of our property rights, including the right to use and the right to exclude others. There's more to property than the right to possession.

In an amicus brief filed with the U.S. Court of Appeals for the D.C. Circuit last week, we argued for Fourth Amendment protection of property rights in data. Recognition of such rights is essential if the protections of the Fourth Amendment are going to make it into the Information Age.

The case arises because the government seized data about the movements of a criminal suspect from his cell phone provider. The government argues that it can do so under the Stored Communications Act, which requires the government to provide "specific and articulable facts showing that there are reasonable grounds to believe that [data] are relevant and material to an ongoing criminal investigation." That's a lower standard than the probable cause standard of the Fourth Amendment.

As we all do, the defendant had a contract with his cell phone provider that required it to share data with others only based on "lawful" or "valid" legal processes. The better reading of that industry-standard contract language is that it gives telecom customers their full right to exclude others from data about them. If you want to take data about us that telecom companies hold for us under contract, you have to get a warrant.

Under the "reasonable expectation of privacy" test, a person doesn't have privacy or a Fourth Amendment interest in information they share with others. But, as we pointed out to the appeals court, the Supreme Court has been moving away from the "reasonable expectation of privacy" test and its step-child, the "third-party doctrine."

The Court of Appeals should put aside doctrine and administer the Fourth Amendment like a law. If there was a seizure---an invasion of a property right, including the right to exclude others from data---that should be reviewed for reasonableness. And the hallmark of reasonableness is getting a warrant.

Speaking of administering the Fourth Amendment, the weird world of data is going to require a deeper understanding of what it means to "search," too. A new law enforcement technique uses advanced data collection and storage techniques to search entire communities before the government knows what it's looking for.

Since January, Baltimore police have been recording all activity in the city from above, using a special, camera-equipped plane. The data collected makes any visible activity available for police to review later. I'm calling it "pre-search," and I've written about it on the Reason blog.

In an ordinary search, you have in mind what you are looking for and you go look for it. If your dog has gone missing in the woods, for example, you take your mental snapshot of the dog and you go into the woods comparing that snapshot to what you see and hear.

Pre-search reverses the process. It takes a snapshot of everything in the woods so that any searcher can quickly and easily find what they later decide to look for.

In this case, it's not the woods. It's every home in Baltimore, and every Baltimorean. Even though the order may be backward, their interests in security from unreasonable search is the same. When this technique gathers information about people's movements and activities in and around homes, the government's collection and use of data should be subject to the Fourth Amendment's constraints.

Pre-search is in use at departments of motor vehicles around the country today. Many are using facial recognition to scan the faces of all applicants and drivers' license holders---and they're doing it without suspicion. Scanning the faces of every driver license holder is a pre-search that sets up innocent people for later digital searching.

The weird world of data gives us a lot to grapple with if we're going to protect our privacy.