Comment by itake

The police used self-powered GPS devices[1] to track criminals. These devices are used in various situations, such as when someone violates parole. The police don’t need to report the violation immediately. Instead, they wait for the person to re-enter their jurisdiction, then catch and arrest them.

Parallel construction wasn't tested, but the means of them catching criminals this way was tested in court.

[0] - https://www.gps.gov/news/2012/01/supremecourt/

[1] - if the device got power from the vehicle, it would be considered "break and entering" and thus would require a warrant.