Earlier quoted context omitted.
> I assume implicit to this statement, is that some heuristic algorithm needs to be created to turn the car off. If you say so. I'm pretty sure just turning the car off if there's nobody sitting in any of the seats and the key is not in the car would be a good start.
"Key not in the car" should be condition enough. I'm having a hard time thinking of a scenario where you NEED to have the key away from the car while the engine is running.
Those people don't need to take the car key with them, but with a keyless car you don't think about the key (that's the entire point of keyless cars). The key is just one of many things in your pockets.