The back of a credit card typically has a magnetic strip that holds information that's visible on the front of the card, like name, account number, and expiration date. A swipe of the card reveals that data. (It can also disclose other information stored on the card, like a frequent flier number.)
Officers often consider possession of a bunch of cards—particularly when accompanied by something like an encoding device—as an indication of identity theft. In order to determine whether the cards are legit, they typically have to swipe them.
Suppose an officer comes by a credit, debit, or gift card honestly—maybe a suspect has consented to a wallet search. Does the officer need some kind of legal justification to scan the card? In other words, is a credit card scan a "search" within the meaning of the Fourth Amendment?
As of 2016, several courts seemed to be of the view that a credit card swipe is not a Fourth Amendment search. These courts have held that an officer doesn't need a warrant or an exception to the warrant requirement in order to scan cards.
In a 2016 federal case, an officer had pulled over a driver for tailgating. The officer reported smelling marijuana when approaching the vehicle. The officer had his dog sniff the car, and the canine alerted to drugs. After a skirmish between officer and motorist, the police searched the car. (United States v. DE L'Isle, No. 15-1316 (8th Cir. 2016).)
The searching officers didn't find any drugs, but they did find a duffle bag holding a big stack of credit, debit, and gift cards. They arrested the driver for assault and resisting arrest. The cards went to U.S. Secret Service agents, who scanned them. The scans revealed that the magnetic strips on the back of the cards had either no account information or stolen information.
The federal government charged the driver with possession of counterfeit and unauthorized access devices. The key legal question was whether the Secret Service agents were allowed to scan the cards.
In concluding that the card-scanning didn't violate the Fourth Amendment, the federal appeals court said that Secret Service didn't commit a privacy trespass: "‘[S]liding a card through a scanner to read virtual data does not involve physically invading a person's' space or property." The court also said that the defendant didn't have a reasonable expectation of privacy in the cards. It pointed out that, to use a credit card, one must transfer the card's information to a seller. In other words, the holder of a card freely discloses the information on it, which means that the information isn't constitutionally protected.
The court considering the case above allowed for the possibility that someone could have a legitimate privacy interest in information that's in a card's magnetic strip. But it found no such interest where all the information in the magnetic strip should have matched the information easily visible on the front of the card.
The decision doesn't, however, mean that an officer can grab your credit card from you without any kind of reason and swipe it. Critical to the opinion was the fact that law enforcement "lawfully possessed" the cards before checking them.
Plus, the decision of that one court doesn't control the decisions of all others. A state court, for example, could rule that the state constitution prohibits warrantless police credit card swipes. (See our note on jurisdictional differences in law.)