Over at SCOTUSblog, I have a long post on the Supreme Court’s pending cell-site case, Carpenter v. United States. My post, “Carpenter and the eyewitness rule,” is part of a SCOTUSblog symposium on the case. My post begins:

One of the most basic ideas in Fourth Amendment law is what you might call the eyewitness rule: The government can always talk to eyewitnesses. If the police find out a bank was robbed, they can go to the bank and interview those who saw the crime occur. They can talk to the bank clerk about what he observed. They can talk to the security guard about what she experienced. They can talk to bank customers about what happened. These interviews, whether voluntary or compelled, don’t trigger the Fourth Amendment. There’s just no Fourth Amendment right to prevent people from talking about what they saw you do.

If the Supreme Court sticks with the eyewitness rule, Carpenter v. United States is an easy case. Cellphone companies are eyewitnesses. Customers use their services and hire the companies to place calls for them. The companies generate records of what they did for their customers. If the police find out a phone was used in the commission of a crime, they can go to the phone company and get its records about how the phone was used. The phone company can be forced to tell the government what it did for its users. Because there’s no Fourth Amendment right to prevent people from telling the police about what they saw you do, getting those records does not trigger the customer’s Fourth Amendment rights.

The challenge for Timothy Carpenter’s lawyers is to get the Supreme Court to carve out an exception to the eyewitness rule. They need to persuade the court that sometimes the law shouldn’t let people talk about what they saw you do without a warrant. The strategy will be to say that the eyewitness rule shouldn’t apply because Carpenter had a “reasonable expectation of privacy” in his location. Because he had a reasonable expectation of privacy, they’ll say, the government should need a warrant to find out what the phone company knows about him.

But this framing is based on a conceptual error. Properly understood, this case has nothing to do with reasonable expectations of privacy. To see why, we need to understand the origins of the reasonable-expectation-of-privacy test.

You can read the whole thing over at SCOTUSblog.