Coolant Level Low indicator

[ungcj]

I have a 2000 4.4 -101,000 miles. I have had to add coolant about 5 times over the last 12,000 miles. It never shows that it is overheating. The bottom of the fill cap (the part that holds the float straw in place) desintigrated. I guess the little plastic parts just went into the tank. I don't know why it desintigrated. I wouldn't have known it except about 1/4 of the ring was attached to the cap. I rubbed it with my finger and it fell off so I know that the other 3/4 must have just gone into the tank. I use the BMW raditor fluid. I bought the car used, but it was always dealer serviced. I usually add about a quart of 50 / 50 mix when it shows low coolant.

I think it could be 3 things, but it could be others:
1. Leak in head gasket? But NO white smoke.
2. Faulty water pump? But no overheating.
3. Stuck Thermostat? But no overheating.

I parked at the airport one time and there was water from the radiator underneath the car (maybe 2 cups), but I have never seen any other signs. It was just that one time.

Any ideas as to what is causing the low fluid?

Thanks in advance!!