The usual causes are :
Power Sounder - the batteries last about 2 years and when they fail, the alarm will go off. The unit is about £50, plus fitting.
Rear glass break sensors : if these get scratched, they appear to the alarm ecu that the window has been broken. They can be repaired for about £15 by windscreen type people.
Interior light/ultrasonic sensors : can give false triggers.
The vehicle battery - if it's getting tired, the alarm is known to malfunction. Always use a good quality battery. GM Dealers offer good deals on batteries, and their units are good quality.
Bonnet switch - known to malfunction in hot weather on darker colour cars (due to thermal expansion!).
They are an absolute pain in the rear. My wife has one, and we have removed/disabled most things as replacing them has had little effect. There is a way to completely disable the factory alarm, but this renders the deadlocks inactive, and could cause you insurace issues - unless you replace it with an aftermarket unit that exceeds the Thatcham Cat 1 spec that the factory alarm meets.
Rob., October 2008