My 124 locks fine but it intermitantly unlocks itself. It's only happened once while I was driving. Many mornings I come out to find the car unlocked. Any suggestions on where to begin my search for a solutions?

Thanks

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1987 300TD; 166,000 mi/267151 Km - had to make room for the dog
1973 220D; mileage ??????
1978 300D; 178,000