Just a thought, it may not affect you but, on the vectra estates, I am sure I have read something about the heated rear screens being part of the alarm system.
They also suffer from exactly the same symptoms.
Iirc, it's because on the estates the ultrasonic sensors do not reach into the boot compartment, so the heated screens were bought into play.
Might be worth looking into...
Hope this helps..

Also, if you plug it into tech 2 as already been said by [MENTION=4]Pecky[/MENTION] you will find out what the last thing that triggered the alarm was.
[MENTION=3]Gairo[/MENTION]