I had the exact same problem with a then new-to-me 1991 300TE. It even locked itself once at the dealer after they pulled the car around after some service. It was caused by an aftermarket alarm system that I didn't even know the car had. After I disconnected the aftermarket alarm, the problem never reoccurred.

__________________
Greg

Current
2005 E320 CDI (50k) W211 with OM648
1999 E300D Turbo (133k) W210 with OM606.962
1997 E300D NA (170k) W210 with OM606.912
1991 300D 2.5 Turbo (199k) W124 with OM602
1990 300D 2.5 Turbo (170k) W124 with OM602

I have had the entire interior out of this car i am 99 percent sure that there is no aftermarket alarm or any aftermarket parts at all (except the radio which i installed). i have had all of the door panels off and have tested the entire vacuum system.

__________________
1987 300D 180k
1985 300SD 330k Helga
1992 300D Lola 269k- Sold
1986 325 coupe Bessie >300k Original engine(ODO Broke) sold
2006 2500HD Druamax Linda- Tows boat and looks pretty in garage thats about it

One thing to mention though i fixed the trunk latch and during the testing the stock alarm system started making the marker lights flash but no horn? i put the key in the driver door and turned it to unlock and the lights stopped flashing

__________________
1987 300D 180k
1985 300SD 330k Helga
1992 300D Lola 269k- Sold
1986 325 coupe Bessie >300k Original engine(ODO Broke) sold
2006 2500HD Druamax Linda- Tows boat and looks pretty in garage thats about it