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.

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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