Just reactivating this one as it's just happened to mine. Alfa sat very happily on the driveway with alarm on. Brought MG B out of the garage and reversed it next to it whereupon the Alfa's alarm went off. Ran back into the house to get the Alfa keys, laughing to myself because the B is always setting alarms off, just usually someone else's.
Alarm won't set properly - keeps going off. Get in Alfa, see that the bonnet light is on. Bonnet is definitely shut. Do some playing about. Light is still on, alarm won't set. Found this thread. Off to remove headlight later and see if I can repair it; failing that I'll have to replace the switch.
Just love the spontaneity of the thing though!
Drink makes you think. Food makes you fatk.