Bad cap - Yep

I had this exact same problem with my 93 400E. I was losing coolant but could not find any evidence of leaking. To make a long story short it ended up being a bad coolant tank cap. When the rubber seal on the cap gets brittle it does not seal well - when the car heats up and the cooling system builds up pressure coolant will escape past the cap and down the drain tube.

On the W124 cars this escaping coolant is a hard thing to spot because there is ANOTHER coolant overflow tank located inside the front fender. (at least there is on a 93 400E) This tank has an obscure drain hole and for some reason my car would only leak while it was running, and even then not too often.

I was finally able to isolate the problem by building a homemade cooling system pressure tester. My cap was leaking at more than 5 PSI pressure. A new cap fixed the problem for good.

Good luck - Tim