alarm siren won't shut up !!

Just got back from a weekend away in the disco towing the caravan. no problems at all,
parked up about an hour later gets in the truck starts it up and drives down the road about 300yards later the siren kicks in!
the car starts and drives the doors lock and unlock off the key fob no problem but the siren won't shut up. I've unplugged it under the bonnet for now everything still works but if I try to plug the siren in it goes off !!!
any body got any ideas ??
Cheers

Faulty ultrasonic detector, unplug.
Faulty door switch, fix.
Faulty bonnet switch, fix.

I had a similar issue last summer which materialised after i bought a trailer. It turned out to be a dodgy earth to my towing harness which for some reason was linked to the alrm by the fuse board. Not sure if that was a factory wire or after market bodge up but i rewired my tow hitch and it's been fine ever since.

would be nice to know what model is...