One problem I could see with all devices like this is the inevitable
situation where someone else has to drive the vehicle, unexpectedly, and
he/she doesn't the right device to do it.

It's snowing out. Your wife needs to move the car from in front of the
garage where it's blocking her car in. She doesn't carry a lot of magnets
around with her, and you're trying to explain on your cell phone why in
freezing weather she needs to wave a magnet right in front of a point midway
between the dome light and the a-pillar...she is not pleased...

Even worse, from this standpoint, is the device that cuts the ignition after
two minutes of driving. Good in theory, but imagine the liability risk when
the unprepared driver gets on the freeway immediately only to have the
engine cut out in the middle lane.

I think one thing that would help is a GPS device, hidden in the car, that
would tell you where it is at all times.