I had the same problem a few weeks ago. I thought at first it was the water pump, but i saw no dried residue from the pump weep hole. Then I followed the dried streak up to the thermostat housing area. And i found a few droplets on the lowest part of the housing. I tried to reseal but it continued to leak. I then replaced it with a spare i luckily had from my old engine. Then it stopped. I had the same symptoms, in that i could drive the car and there was no leaks. Then let it sit for a while. I would go out at lunch time from work and take a look, and i could see the leak beginning. Then it would stop after it leaked a certain amt. I suspect it had something to do with when the thermostat would release while the engine was cooling and it must have created some pressure and released it out the weakest point in the system.
1988 190e 2.3 with 91 engine