1997 E300D (210.020, OM 606.912), 150K miles

Vehicle has starting loosing coolant; only visible sign of coolant loss is at the front of the cylinder head... a very small quantity of coolant can be observed forming on the short lip of the cylinder head gasket that extends out from the front of the cylinder head. Coolant subsequently drips on water pump and upper timing chain cover and evaporates, leaving stains.

Engine starts readily and runs without obvious faults. No coolant is present in the motor oil and no oil is present in the coolant expansion tank.

There are no coolant passages at the very front of the head gasket.

Any insight as to where the coolant leak might be?