A city near me has installed a device that tracks vehicles based on their tpms (Tire Pressure Monitoring System) sensors.
All cars after 2008 in the US have TPMS. Inside the tire, integrated with the vale stem, are little pressure sensors with a radio that broadcasts on the 315Mhz band. Each one uses a slightly different frequency so that your vehicle can tell which of the four tires is low.
So each vehicle in the US made after 2008 has four unique radio signals being broadcast from it, and now there are police departments with equipment that can track those signals, and can assign each car a signature based on the frequencies the sensors are broadcasting on.