Ok, a few months ago, my cooper was having an overheating problem. Got those issues fixed, but ended up replacing the head gasket because i noticed that small amounts of coolant in the oil. I could not find anything that looked obviously wrong with the head, or the gasket, no obvious failures.
Took the head to a shop, they measured 3 thousands of warp in the head. They said it was within spec, skimmed it and I reinstalled with a new head gasket.
The car runs great now. no codes, idle is really smooth and the engine seems strong.
But, I am losing coolant. I cannot find a leak anywhere. After I park the car, the driveway is clean, no water, no oil, no nothing, no coolant smell, no leaks in the cab. In two weeks, i went through about 3/4 gallon of coolant.
There are small globs of moisture on the oil filler cap, and the dipstick (possibly left over from before the repair) but the oil looks pretty clean except for these globs, it certainly doesn't look like there is an extra 3/4 gallon of fluid in there.
These are my thought:
1. head was not in spec and water is going from the water jacket into the oil (seems like there would be a lot of water in the engine if that were the case.)
2. maybe the little bit of moisture is left over from before the repair and it is not being vented out of the valve cover? maybe my valve is plugged? How do I test the vent valve? That would mean the coolant is going somewhere else. I have a new overflow tank, and I cant find any other obvious leaks.

Any other ideas I should check?

 

Look at the coolant tank....it can leak along the seam...
Next, look at the thermostat gasket+housing.... The gasket (molded into the thermostat) leaks, and the housing can sometimes warp overtime.
Next...look closely at the RADIATOR.... It has an aluminum core, with plastic side tanks...the tanks are heat set to tanks...and the plastic ages and cracks...causing leaks..and at the age of the car, every hose is suspect...so look around...
The water pump is pretty hidden, so it would be hard too look at the weaphole for a seal, but you might see dried residue in the area.

 

Along with Zippy's suggestions; you could still be loosing water in the head if the water is just leaking into the cylinder and being burned off. If so, you should see white smoke/steam out the exhaust, especially on startup. I suspect this is not the case since you had the head off.
Some leaks tend to hit hot engine parts and evaporate leaving only white residue.
I would not worry about small globs of water on oil cap.

 

Ok, I drained the oil, got right at 4 quarts without doing the filter, so there is not much in the way of any extra fluid in there. Also, it looked very much like oil, didn't appear to have any water in it. The only evidence of any water is the few little green globs on the filler cap, and the dip stick.
I found a small leak in the radiator, but it seems to be a very small drip, sure doesn't seem to be enough to lose a gallon over two weeks.
No white smoke, and I can't find any other leaks. I'll patch the radiator and see if that was the culprit.

 

How do you plan to 'patch the radiator?'

 

Is it possible it wasn't bleed correctly when the head was replaced and you have slowly been displacing trapped air?

Try bleeding it and then monitor level again...

 

A TINY leak in a radiator will spray pretty good with the car running at temp...remember,it is pressured at 7-15 PSI when hot to increase the boiling point of the coolant....you can watch it...hose it off...but it's only gonna get worse.