2008 Devon Sprinter Alarm Fault

Having deactivated the alarm system with the fob, the lights flashed etc, I then opened the drivers door and the horn went off. After a few minutes it cut out. I put key in ignition, it went off again. So whatever I did, open either of the cab doors, key in ignition, open bonnet, the horn was activated. Pulled out the 15A horn fuse, horn still blaring so crawled under van and disconnected the horn. Drove a few days lated to get MOT and service (not at a Mercedes dealer) and explained situation. Horn was reconnected by them and no fault could be found by the engineer so passed MOT. Two weeks later the same thing occurred, I immediately disconnected horn again. As on both occasions it had rained previous day, so I am wondering if damp is getting in to something? The Cobra alarm system fitted by AS seems to be operating correctly and even when accidentally set off on previous occasions I have never heard the Sprinter horn go off! What’s going on? Any clues please? Thank you.