the other thing is - it is not the car that makes the sound but something else... like you said - it could be that the previous owner (if there was one), has dropped a kiddie watch in between the seats and that watch is beeping at random times...

try to correlate that sound to something that you or the car do - if the beeps happen at the top of an hour, or if the beeps happen once you are over certain speed, if the beeps happen when the ambient temperature is at certain level... in absence of those factors, and if the beeps are not that of the car (there are three distinctive "gong" sounds), then it is something either aftermarket, or totally unrelated to the vehicle...