In all probabilities the old siren will be toast, which is what is causing it to go off. You can replace it with a Benz siren that beeps when you lock/unlock the car, for about $90. I can get the part number if you need it.
I am assuming this because the car still locks/unlocks with the fob.
If you haven't already, do yourself a favor and remove the plug from the passenger side trunk recess (from below)
Another cause of the alarm going off at random, is the central locking pump being submerged in water, due to a leak into the trunk.
Mine got full of water when I was out of town, sounded the alarm, and flashed the lights for so long that that the battery eventually went flat, locking me out.
Anyway it's piece of mind to check the trunk well for water, and if none present, a change of the siren should be all you need.