I came across the Byonics website today.

While I cannot understand why anyone would want to advertise the location of their car via a continuous beacon on a 2m fm radio, especially when the car is unattended in a car park, the devices offered by Byonics look interesting and may have other uses for the enterprising electronics or amateur radio experimenter.

The APO3 automatic power-off device is one example.  If your battery voltage drops below a predetermined level, it turns off the power to the radio.