The power sounder is a separate ( piercing )alarm that activates if the battery is disconnected the main alarm ( horn lights etc ) is supplied via the main car battery it is to stop a car thief disconnecting the battery and towing away a new car.
The power sounder unit is located behind the inner mudguard in the front passenger side wheel arch to the rear near the bulkhead. peel back the plastic mudguard to access the unit.
Turn on the ignition, this stops the unit sounding off and remove the PS unit ( 2 torx screws ) take off the plug connector and throw the unit away (No one is going to steal a mk4 Astra ). put the connector plug in a plastic bag to prevent it getting wet and tuck it up out of the way.
The unit has ni cad battery's which start to fail in time they leak acid onto the circuit, this is the cause of the false alarms.
The main alarm will still function as normal the immobilizer will still work as normal nothing else will be effected. You do not have to have anything reprogrammed or recoded at a Vauxhall dealer.
This is a free solution be thankful this does not happen very often. Sleep well dear friends.