This could be fixed with an on/off button on the fobs, no? It takes some of the convenience out of the fob, but it would still be somewhat easier than fiddling

This could be fixed with an on/off button on the fobs, no? It takes some of the convenience out of the fob, but it would still be somewhat easier than fiddling around with a physical key, though quite a bit of convenience is lost. Another way would be to attach the key to something like a smartphone. The car finds it location via GPS and stores it on a server somewhere. If the fob receives a signal from what it thinks is the car, it asks the smartphone to confirm its location is near the car's location. The fall-back in case the smartphone is out of battery or the car did not post its location would be this vulnerable method where the fob simply responds if it has received the message from the car.