The U.S. Supreme Court on Tuesday went looking for a clear rule to apply to police searches of rental cars whose drivers are not authorized on the rental agreement to be behind the wheel. At the end of an hour-long argument, the justices were still searching.

“What worries me is what's our rule going to be,” said Justice Stephen Breyer. “Are we going to have a [Fourth Amendment] rule for car rental cases?”

The case, Byrd v. United States, reads like a law school exam hypothetical: Does the driver who was stopped—who is only behind the wheel with the permission of the authorized driver—have a reasonable expectation of privacy in the vehicle?

The U.S. Court of Appeals for the Third Circuit concluded last year that one driver, Terrence Byrd, did not have a privacy expectation after a highway stop near Harrisburg, Pennsylvania. Byrd, according to police, consented to a search of the car and its locked trunk. The officers also reportedly told Byrd they didn't need his consent since he was not an authorized driver on the rental agreement. Byrd said his fiancée, who was the authorized driver, gave him permission to drive the rental car. In searching the trunk, police found body armor and heroin.

During arguments, Byrd's pro bono lawyer, Robert Loeb of Orrick, Herrington & Sutcliffe, faced a web of hypothetical situations posed by the justices—from rental car hijackers to fathers giving permission to sons to drive the family car to car rental agreements permitting police searches.

“If you're given permission by the renter to store your personal items in the car, you have a reasonable expectation of privacy,” Loeb argued. “Unless it's a stolen car or a criminal trespass, there is the ability to invoke the Fourth Amendment.”

But Eric Feigin, an assistant to the solicitor general, pressed a hard-line approach.

“Byrd has to justify an affirmative connection to the rental car,” Feigin countered. “He can't do it. As an unauthorized driver, he has no connection to the car,” and so no reasonable expectation of privacy in it. And, he added, Byrd's fiancée had no authority to give him permission to drive the rental.

Justice Sonia Sotomayor said she worried that under the government's approach, police would have incentive to stop every rental car and fully search those in which there is an unauthorized driver. Justice Elena Kagan noted that a violation of the authorized driver requirement—“not uncommon,” she said—goes to insurance liability, not the Fourth Amendment.

Chief Justice John Roberts Jr. said: “It's important to give clear guidance to the police. If it's a rental car, look at the agreement. If you're not authorized to drive, that's it.”

Even if the rental agreement is not in the rental car, Feigin replied, “We'd make the same argument.”

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