I'd say it's a head gasket, could also be a cracked head. My 75 Chevy had a
baffling problem. Fill the cooling system cold, run engine, no leaks, go
for a drive, check coolant after cooldown, coolant low. Scrubbed engine,
never a drop outside of block,
never a puddle under car. Took heads off to do valve job, and discovered
hairline crack between valve pocket, and coolant passage.