Not a perfect solution. Keys are sometimes set down in the vehicle before starting. A time based rule (motion within last x) won't help because sitting in a no

Not a perfect solution. Keys are sometimes set down in the vehicle before starting. A time based rule (motion within last x) won't help because sitting in a non-running car is something that happens often enough that keys not working would be a problem. Also, someone could steal your car while you have your keys in your pocket inside your house, or walking around a store.