1985 745t

Getting medium to slow speed coolant loss and am unable to figure out where the coolant is going. The coolant will be filled to the max level and will then drain down over the course of 2-3 weeks. Long trips at high speed tend to accelerate the loss, but it seems to happen regardless of driving conditions. However, it will never drain down to nothing and seems to stop at a particular level just below the expansion tanks "reservoir."

Some of the things I've looked at include the following:

1) Head Gasket/Oil
Coolant shows no sign of oil or contamination and oil is clear of any condensation, sop I've ruled out a headgasket between those two items. Spark plugs check out fine with perfect color and no condensation.

2) Expansion Tank Cap
Coolant cap is new 75kpa cap and shows no sign of any leak or failure (no trademark residue on exp. tank).

3) Head Gasket/Exhaust
Exhaust sometimes shows a little steam at start-up (and more when cold) but it goes away quickly as it warms up. Exhaust smells "slightly" sweet, but I don't really have a baseline to compare it with. No coolant or excess water at the exhaust though, so I'm not sure about a leak from the coolant to the exhaust.

4) Coolant System Leak
Coolant system has been fairly closely inspected and shows no sign of a leak at the obvious points like drain cock, temp. sensor/sender, water pump, radiator, heater valve, core (no steam, no liquid on floor).

Basically I've done everything imagineable to find this leak and can't figure out where the hell teh coolant is going. I do know that pressure seems to make it drain faster; it was fine with an old leaking cap I had, but once i replaced it I started seeing this bottoming out effect.

Any help is greatly appreciated.

Thanks,
rt