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This is an engine network malfunction that happens when the car is running. The ignition switch signal to shut off the engine never makes it to the engine computer. Typically the gear shifter is the cause of the problem. It creates "noise" on the network communication lines not allowing some signals to get through. I know this from experience because I have had it happen on several S class models around your model year. Of course a definitive diagnosis would require some extensive network testing while the problem is happening. The shifter can only be ordered from a dealership and has to be programmed with a mercedes computer which can get expensive. Expect in the $1000-1200 range.