Im not to familiar with the original alarm, my car never even came with the master key that uses the remote central locking.
Id start with finding the with sensor for the alarm, most commonly placed inside the cabin in or around the central console where your handbrake is. If its an adjustable sensor (probably not) it could be turned to the most sensitive setting, or it could of come loose and is hanging by the wire and in turn sending wrong signals. That failing it could be a short in the circuit somewhere.
Electrics really aint my strong point and tend to vere away from them!!!
The ONLY Hellrot Red 323 in the world, emmm except for the other few thousand dotted around the world.
there are many sensors ; try disconnecting the internal ultrasonic unit in the headlining failing this the best bet is to get a garage with obc reader to locate the faulty component and iether replace yorself or disconnect for example bonnet closed sensor.