This is not to replace a car alarm.
This is for when the condition arises that your car has actually been stolen but you the owner are not aware of it.
Install a mechanism that automatically flips your plates to read "ALL POLICE ARE B*STARDS!" (or another attention-grabbing phrase or word of your
choice).
The police are bound to stop the thief over other stolen cars, which haven't yet been reported yet.
The mechanism would be automatically kick in as soon as the ignition is started, but would also have a secret over-ride button for when the car is being driven not-stolen.
Just don't forgot to hit the over-ride button; or you'll be breaking several laws.