What does a coolant leak smell like with MB coolant?
 

Every once in a while, my 95 S600 windshield fogs up near the base of the windshield (inside surface). When I turn on the defrost, the windshield really fogs up fast, but after 10 or so seconds it clears up fast. This doesn't seem normal to me. I am wondering if I have a small coolant leak in the heater core. I'm used to the green coolant used in American vehicles, and my nose is sensitive enough to smell small leaks with this stuff. The MB coolant does not seem to have that "sweet-ish" smell. The car was pressure-tested, and passed requirements. I know that a small leak might not show up during this test. Can anyone tell me if a sniff-test is possible, or are there any other recommendations?

p.s. There is no sign off coolant in the passenger compartment. I disassembled the area to see if the condensation drain lines were working properly, and they are. Everything looked OK in there.

I would just remove the rad cap (WITH ENGINE COLD!) and take a whiff, that will tell you what it smells like. If it were coolant it should be a kind of greasy film on the windshield.

I always thought of it as a hospital antiseptic smell.

I think your car is behaving normally. I also sometimes get a temporary fog when I turn on the defrost or a/c. This is probably because the evaporator box is wet with condensation from the previous day's a/c use.