vw bora alarm

I have a problem with my '99 VW Bora alarm. It goes off all by itself, when ever it wants to. Sometimes every 5 minutes and sometimes nothing happens for 5 days and then for no reason the alarm would go off again. ( the car has factory alarm, and the central lock and alarm is all in one key). I took my car to a bodyshop - they couldnt find any electrical problems and all the sensors seem to be ok as well.
What could be wrong with the alarm? Thank you for help!

The only thing I can add to checking all the sensors it to pull back the rubber boot at each door that the wiring harness travels through to the body & check for frayed/broken wires.