Many cars now have remote locking and unlocking on a keyfob.
Typically these flash the headlights and make an audible 'chirp'.
Sometimes people use this function to locate their car in a car-park or similar.
Obviously this is not ideal, because there is the risk of leaving the car unlocked,
and if many people are doing this simultaneously you might accidentally get into someone elses car.
I propose an additional button on the keyfob, which plays a short user-configurable 'ringtone' and doesn't unlock the doors.
I suggest offering first refusal on the aftermarket installation franchise to the Carphone Warehouse so they can regain some dignity.