Morning Doc, I had a crack in the seating surface of one intake valve.
The motor would burn the minimum amount of water which entered the cylider every time that one intake valve was open during operation. Causing it to slightly overheat and lose water.
It was pure coincidence that over one winter that very valve was open and let water in.
I would guess you could track this failure by running the motor to pressurize the coolant system, stop the engine and turn it by hand to open one intake valve at a time and check the cyliders for water.
This could be a bit elaborate, because once the pressure in the coolant system is gone, the hairline crack would not be wide enough to let water through.
Or in fact, just pressurize the system like checking for radiator leaks should work.
I also once had a massive
oil leak when I mounted a set of used heads with threads in the intake channel to hold down a rev kit. I didn't see those threads, even though it was very obvious.
Well, we are always clever afterwards :-)