The problem is that about 12-24 hours after the engine is shut off, the pressure forces coolant out somewhere near the water pump or thermostat area - I can't tell exactly. (I'm guessing the water pump seals.) When the engine is hot it doesn't leak. I'm speculating that when the metal w/p body cools, the seals shrink a tad and allow seepage. After all, the system is designed to hold 1.4 bar at full operating temp, not at ambient. If I find it is coming from the w/p I'll probably replace it anyway.
However, I don't feel the real problem is the leak (although that's not good), I think it's the fact that there is high pressure when cold - which is NOT normal! Usually, when the engine cools off the system is under slight vacuum. The upper radiator hose may even be "compressed" a little bit - but there is NEVER any pressure present when cold.
Almost the entire cooling system was replaced ~3 years ago - new radiator, water pump, fan clutch, thermostat, all 5 hoses in the engine compartment, new plastic reservoir/tank, radiator cap, and factory MB coolant installed w/RedLine Water Wetter. It's been doing great up until a few weeks ago.
[Side note: This proved the dealer to be totally wrong in their claim that "if it's not in the red, it's normal operation." What a load of crap. The car used to run at 105-115C in the summer, and after replacing all the above it now refuses to ever exceed 90-95 except under the most brutal conditions (i.e., 110F ambient, climbing a hill with the A/C on). The problem in my case was a bad radiator AND a bad fan clutch. I changed the other stuff as preventive maintenance. My experience with several other 1987 300D's is the same - they should never get over 90-95 most of the time, and 105+ indicates a serious problem.]
1987 300D - 228kmi
1987 300D - 234kmi
1984 300D - 205kmi