[Chucksta]

Or... test it the other way.. pressurize the cooling system, if it leaks down, but doesn't leak out of the car...

After that test, if it leaks pressure, but there's no visible leak of coolant, you'll probably get a good whiff of it out the tail pipe when you start her back up.

The rough explanation would be that when you were running, the head gasket leaked just enough to push coolant into the reservoir. But, when the system cooled, instead of sucking coolant back into the system, it sucked air from the leak in the head gasket from the cylinder back into the cooling system, leaving the expelled coolant in the overflow.