One other thing to try is closing all the air vents before locking the car, in case warm air currents are triggering the sensor. Oh, also make sure the seat belts are reeled right back in, in case any slight movement after locking is triggering the alarm.
Will check the bonnet catch as suggested and try closing the air vents. I am certain its not seatbelts as after the first time i double checked them, I used to have an impreza that was notorious for the seatbelts moving and setting off the alarm!
It has been suggested that it could be vibrations from vehicles passing it but I don't think that will do it as it has never done it before - however next doors huge van reversed next to it this morning and that set it off? I have shook the car by its roof bars and had it all rocking and this didn't trigger the alarm.
I have shook the car by its roof bars and had it all rocking and this didn't trigger the alarm.
This of course isn't the same as the windows getting vibrated at a certain frequency by next doors huge van.
Several years ago there was only one thing that falsely set off my alarm on a previous car; that was a neighbours mk3 Ford Escort XR3i as it drove by. Nothing else ever set it off.
I *doubt* very much if the internal sensors are picking up seatbelt movement of airflow from the vents. From previous experience of Vauxhall alarms, the sensors aren't that sensitive. Yes, they will pick up a window breaking by air pressure difference, but if the windows are wide open, you have to frantically wave your arm through the window before the sensors will pick up the movement.
I've got a 2004 Zafira. You can turn off the interior sensors by pressing the 2 interior light buttons (above the mirror) before exiting and then setting the alarm within 10 seconds. A light comes on in the central dash for those 10 seconds.
Had the same problem with night time alarms a few weeks ago. Disabling the interior sensors had no effect. I took it into to the dealer under warranty and they claim to have 'fitted a new alarm siren'. This has fixed it.
Failure of the "powersounder" is a common problem on Vauxhalls with factory-fit alarms. The back-up battery within the siren unit fails. 4 beeps are given when switching off the engine when this battery is starting to fail. If the powersounder isn't replaced, the alarm is eventually triggered, after a few weeks.
I have to replace the powersounder on my 2000 Astra every 2 years. It really ought to be on the service schedule but Vauxhall claim they sometimes last longer!
The unit is about £55 and can be fitted diy if you adhere strictly to the "how-to" on www.astrasport.co.uk but it may be best to leave to a Vauxhall dealer as faulty fitting will need "re-programming" at the dealer anyway.