There are a couple of other simpler possibilities as well. It could be a door switch or a hood or trunk switch. The factory alarm is set to go off if any door, the trunk or hood is opened while the system is armed. So, if you have a switch that is flaking out, or isn't lined up as well as it used to be then that could be your problem.

Check the one under your hood first. On an S-class (not sure about your car) it is mounted to the firewall using two screws. I could see those getting loose and making the switch sag. You could put a small o-ring around the shaft of the switch and slide it all the way to the base. Then, when you close the hood the o-ring will slide up the shaft as far as the switch went in. This will tell you how far the switch is actually getting pushed in. The same trick will work on the doors too.

Worth a check....


-tp