its probably simple, not much can go wrong with these engines, and you don't have milky oil or oily coolant so don't worry, at the rate of coolant lose you mentioned, it is definitely going outside the engine somewhere and not inside ....(i think you have an m103 by the way)

see what the mechanic says, but i bet it is either the water pump seal, or a hidden radiator leak somewhere,,,,

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1993 300E, 2.8 M104 .....