'nut, he isn't loosing any coolant.
what you describe would indicate a failed headgasket, but his scenario could be a cracked head, or a very minor leak in he headgasket that is only comprimised by the high pressure of combustion 160-180PSI
the cooling system is 15ish PSI?
So the leak may be combustion gases going FROM the cylinder into the cooling system due to the 160-180PSI combustion pressures.
could be the 15ish PSI from the cooling system isn't pushing coolant back into the cylinder because the leak isn't "that bad".
My SVO mustang did this when I ran 27+PSI boost, it cracked the head.. but wasn't loosing or swapping any fluids.
Only combustion gases from the combustion stroke were entering the cooling system.
The problem was found by using a dye test that confirmed combustion gases in the cooling system, pulled the head and had it checked and it was indeed cracked.