i had this problem in my sprint car engine a couple of years ago. It never ran hot, but would loose a little water and a few days after racing when I removed the radiator cap it would release pressure. It ended up being a hairline crack in one of the cylinders.
I pressure tested the cooling system and could not find anything. It was such a fine crack it would only open up when the motor was up to temp. Combustion gasses in the cooling system is the only thing that will keep the radiator pressurized.