Here’s an interesting Fourth Amendment question decided by the 8th Circuit last year in United States v. Cowan, 674 F.3d 947 (8th Cir. 2012): If the police have a suspect’s car keys, and the key fob has an electronic button that sets off the car’s alarm when pressed, can the police press the button to set off the alarm and therefore identify which car belongs to the suspect? In Cowan, the officers had seized the suspect’s keys, and they suspected that the suspect’s car was nearby and might contain drugs. To identify which car belonged to the suspect, they walked around the streets near his home pressing the “alarm” button until a specific car’s alarm siren was triggered. They then brought in a drug-sniffing dog to sniff that car, the dog alerted, and a search of the car revealed crack cocaine inside.
The Eighth Circuit held that pressing the alarm button on the key fob did not violate a reasonable expectation of privacy because it did not reveal any private information — an approach to the REP test I have called the “private facts” model:
Cowan did not have a reasonable expectation of privacy in the identity of his car. The Supreme Court has noted “the diminished expectation of privacy in an automobile,” Knotts, 460 U.S. at 281, and that “[a] car has little capacity for escaping public scrutiny. It travels public thoroughfares where its occupants and its contents are in plain view.” Cardwell v. Lewis, 417 U.S. 583, 590 (1974).3 If Cowan’s car were in Chicago or otherwise not present, the fob would have disclosed nothing. If the keys and car belonged to someone else and Cowan had no possessory interest in the vehicle, Cowan had no expectation of privacy in the vehicle. If the keys belonged to a car in the apartment’s associated parking area, the fob merely would identify the vehicle. The officers could have obtained the identification information by conducting a background check on the car’s license plates or vehicle identification number or placing the car under surveillance and waiting for the owner or for an abandonment
Cowan argues his privacy interest was in the key fob’s electronic code. The officers did not attempt to discover the code. Pressing the alarm button on the key fob was a way to identify the car and did not tell officers anything about the fob’s code or the car’s contents.
The Court also ruled that there was no trespass under Jones, as “the mere transmission of electric signals” to identify the car is not a trespass.