Sorry but it does sound like the head gasket has gone.
It may be that the gasket has failed towards an outer water jacket and the fluid leaks, when cold, then when the engine has warmed up the head/block both expand temporarily sealing the leakage until the engine cools before.
Unusual but most likely.
You could purchase some UV dye used in A/C gas leak detection, refill the coolant and add an egg cup full of the dye and at least the dye will leave a visible trace.
Hope that helps.
Good Luck!