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In the case I have seen, the 4-stroke cylinder on the combustion stroke was blowing gas past the gasket and a water channel. In the process a little water was vapourized and ejected. It only happened at running speed and the exit point was well hidden under the exhaust manifold, so it took an age to detect.
At running speed, the cooling assembly was slightly pressurised and a little above normal boiling point, so taking the radiator cap off was not that advisable. It was a vehicle not a boat engine.
Only speculation: in your case, if the gas is blown by a compression stroke cylinder, past the water channel, into an exhaust stroke cylinder, the water would apparently vanish entirely.