Comment by Animats

That's how classic Lojack worked. Lojack is a car theft detection device. It's not GPS based. It's hidden somewhere on the vehicle and connected to power. It listens to FM broadcast radio stations until it finds one with the Lojack subcarrier, something Lojack licensed from radio stations in most cities. It decodes the subcarrier, listening for serial numbers of Lojack units that have been stolen. Then it starts transmitting. It doesn't transmit location, just its serial number. Police cars equipped with Lojack receivers get an indication of direction and distance. Lojack had an effort to equip police cars with the equipment. At peak, LoJack had coverage in parts of 29 states.

It was very effective. 98% recovery rate.