The doors on my 90 300E would lock by itself occasionally when I'm driving, then seconds later the alarm would be triggered and the horn would go on and off. I would have to stop by the side of the road, use the key to unlock the door to turn off the alarm.
I took the car to several mechanics, none was able to identify the cause. However, last winter the local dealer discovered a leak in my right rear light assembly, which was causing rainwater to fill up in the trunk. When they fixed the seal and dried the trunk the problem went away for several months. Recently however, the problem seems to have resurfaced, albeit infrequently. In addition, the External Lamp Failure light comes on when I shift out of Park -- as far as I can tell, all the lights are working.