Only problem then is they have probably already wrecked your door!I've wondered about a loud as hell horn that simply operates on the door opening. Needs to be unlocked with a beeper like your car before you open the door.
Deafen the *****.
My pirs are fitted behind the featherboards. The only thing you can see is a slot about 10mm high and 50mm wide, these are positioned under the wide part at the bottom of the board above. They are virtually invisible but each has a field of view of about 30 degrees side to side, and a range of about 12-15 feet. They will only detect something that's between roughly 4 and 5 feet off the ground, so the dogs, Muntjac etc don't set them off. These protect the side accessible from my neighbour's garden, the rest is protected by beams. As I say the beauty is that however you approach the building you can't get right up to it without setting off the alarm, so hopefully they piss off before doing any damage, and without knowing why or how they set it off.