Does the front doors and the trunk arm and disarm the factory system consistantly. It is important that all three work properly and if one doesn't then I would disconnect that switch and check the wiring in that door or in the trunk. I mentioned before I have seen this exact problem and it was caused by wires in the rt door were grounding out setting the alarm intermittently. The problem cannot be a sensor as the sensor do not set the alarm and in order for the alarm to false it first has to be set. The thing that is weird about the early factory alarm is that it can arm even if the car is running. There is no ignition kill so it won't stop the car from starting or kill it when it is running. I think you can lick the problem if you go about it as I have outlined.
The Mercedes Specialists Since 1986