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The only other possibility that I could think of would be that the water pump housing is starting to leak. Whether it's from the impeller or from mating surface would be difficult to say. It wouldn't have to be a large leak. When I did my rebuild that was an issue I had to overcome.
Other possibilities might be cracked coolant lines that run into the heater housing (let alone the heater might be leaking as well). Basically you'll need to track down to see if there's a pin hole leak anywhere in the car.
Or you could just shove a borescope (or a mirror) into the exhaust ports and see if there's any coolant pooling.
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