I noticed you said interior was disconnected in an earlier thread/post when DH described.
Are you sure it hasn't been bumped parked out on the road, you wouldn't know unless there was noticeable damage, you not being there ?
Yes, heavy lorries/traffic could rock it, mine went off once in the yard in heavy wind, I moved to a different position where it wasn't sideways on, which solved it. I'd also left a window slightly down so had the horns AND siren blaring. Since then, had just the horns when a fly was inside after a day parked with widows wide open, then closed for the night.