Some people will take an old smart phone they aren’t using anymore, and hard-wire its charger into a hidden area of the car. That keeps the phone charged up and turned on. Then they use the find-my-phone feature (or Lookout if it’s an Android that doesn’t have one built in) to locate the vehicle if it’s been stolen.
The obvious disadvantage here is that if the thief takes it somewhere without good cell reception, you’re out of luck, though you can look at the location history to at least see where the thief was going before cell reception was lost (in, for instance, an underground garage).