I had the same problem on a '99 ML430. Alarm would go off for no reason. They said it was the sensor that detects the vehicle has been lifted (like towing) which is linked into the security system. I don't recall if they reprogrammed or diabled it.

Is the horn that sound the 2 horns mounted in front of the wheel wheels of the one near the brake fluid reservoir. If near the reservoir, I'd personally just disconnect it until I could resolve the issue. Have you tried parking in a different position (pointed down vs up, etc.)

Good luck and be sure to let us know the outcome.

__________________
Jim