Here's my story.
Bought my 85 500SEL and it came with two keys, both MB, one with red dot (flat key is M.I.A)
Both keys lock the vehicle completely, but only the red dot sets the alarm. At the same time the sensor chip in the driver's door was (supposedly) worn out from years of use.
Using the red-dot key:
If I lock the drivers door, I could unlock anywhere and the alarm did not go off.
If I lock the passenger side, or trunk, and then unlock the drivers side, the alarm did go off.
If I lock the pass/trunk, and unlock the pass/trunk the alarm was set, but did not go off.
Using the plain key:
No matter what, the alarm never went off.
Using the red-dot on the pass/trunk locks to set the alarm, and if my wife came later with the other plain key the alarm always went off regardless of which door she used.
This all seems to confirm that
1) The red-dot key is the only one to set/disable the alarm
2) The sensor chip in my drivers side door lock is worn.