With a closed system like the coolant, there are precious few placed to be losing coolant, but I clearly have a serious leak yet I can find no pools of fluid under the car or on the floorpans. I'll attack this problem first thing in the morning, but I wanted to solicit some comments to back up my reasoning and maybe take a few short cuts.

A little history.
The car has a new B20E, with 120,000 miles and no history of serious oil or coolant losses. I was suffering some coolant loss when the old B18D was still in, but it was not nearly as severe. The B18 seemed to lose coolant much faster when the heater was on. There *was* a crack in the short "L" hose from the head to the heater core, but that hose was replaced shortly before the B18 died from oil pump failure. The B20E's head gasket was replaced a couple weeks ago after failure due to insufficient coolant (my fault). It spewed white smoke out the tailpipe when it got warm. The motor was pulled, the head gasket was replaced, torqued and re-torqued to spec in a multiple day process (as recommended). I added two gallons of dexcool and distilled water through top radiator head hole and later into the expansion tank.

Anyway, after taking care of my PCV system problem, I noticed that after topping off the coolant at least three times, I seem to be losing it very rapidly. This is troublesome because there is no pool of green or orange fluid under the car or in the car. It's like the coolant is escaping into extra-dimensional space. I can't find it. I was concerned that it was escaping into the oil pan, but my fluid level in consistent and shows no orange or greenish tint, nor does it smell like coolant. The oil in the head/rocker is not greyish or creamy. I thought that it was maybe leaking into the cylinders and being burned, but the car starts easily and does not spew white smoke out the rear. Yet I'm definitely losing coolant rapidly.

I placed a dry newspaper under the tailpipe to look for coolant dripping out or being burned out, but aside from some slight spotting, there's no indication that I'm losing coolant there.

After shutting down the motor, I checked and my coolant had dropped to half-way on the expansion tank. I made a slight pen mark to track the level and chatted with a passing neighbor for about 10 minutes. I came back to find that the expansion tank has lost 1/4 of it's fluid w/o the motor even being turned on.
I inspected under the car for fluid and there was none. WTF?!

Tomorrow I plan to replaced the expansion tank and hose with a newer equipment, but aside from pulling my whole heater unit, I'm not sure where else to check. All the hoses and known valves are tight and supple with no pools of fluid under them. The car is just not getting hot enough to dry *all* of that coolant w/o me noticing.

This is very confusing and concerning and not at all timely considering the time I just devoted to tracking down and finally solving that PCV problem.

Any Ideas as to what's going on here?