Vanishing coolant

[Mickeyrouse]

Coolant is disappearing from my engine. It is a Westerbeke 55B,which is a marinized Isuzu 4LE2. I have replaced the pressure cap. I have gone over all hose connections and heat exchanger end cap repeatedly, yet find no evidence of drips. The drip pan under the engine is dry. The engine oil is clear, with no sludge or other sign of coolant getting in to the lube system. Coolant doesn't appear oily either. Since the engine only has about 150 hours since a new head gasket was installed, this lack of sign doesn't (at least to me) point to gasket issues. I've tried to run a pressure test with pressure test kits available at auto parts stores, but the 30-odd adapters don't include the type that fits my header tank.
I'm out of ideas. The only I can think of left is to pull the heat exchanger and have it tested, but that won't reveal any hidden leaks elsewhere in the system. Anybody have any new ones?