I've read of 60x water pumps leaking without any indication in the weep hole visible from above. There is something of a drain tube at the bottom of the water pump. I don't know what it's for. I would have a 60x expert, they're few and far between, check the pump.

I'd love to have you pull the cylinder head to give the rest of us 3.5-ers a point of reference but I don't think your coolant loss warrants that as a first step.

Sixto
93 300SD