This is for a 1983 240D again.

I have a slow leak(a couple of cups a week) in the engine coolant. This is my daughters' car. I assumed it was the water pump, but there does not seem to be any leakage there. The radiator cap is new. There is no oil in the coolant or coolant in the oil. I cannot find any coolant in the passenger compartment and I do not see any leak in the engine compartment. There does not seem to be any hissing or other unusual noises when I shut the car off.

Anybody have another guess as to where it might be leaking?

I had the same problem a year ago and finally it came out a leakage of coolant from the connection between the coolant hose and the engine compartment.Simply replace the hose and it gone.It was so slow and not noticable because it didn't leak when the engine on but just start to leak after used. Just my experience and hope this helps...

Had a similar problem last week. It turned out to be a cracked inlet to the radiator where the hose enters the radiator from the water pump. Had to get the tanks replaced with metal bands inside. It was causing the temp to rise just slightly and putting coolant around the edges of the hose, where it enters the radiator. It was not noticeable either. It was leaking underneath hose clamp, little by little. Check hoses and do coolant flush. Then, you should find the leak hopefully.