As it passes by a license plate reader, the camera records the car, which can be linked through vehicle records to its registered owner. Weeks later, one of the same device signals appears alongside a different car connected to an investigation. SignalTrace, a system marketed by the security company Leonardo, is designed to work alongside automatic license plate readers. The company says it can recognize groups of consumer devices that regularly move together, then associate them with license plate records and time-stamped locations. Even a correct match between a device and a vehicle does not establish who carried it on a particular day.