One of the causes of disappearing coolant which re-discovered every few years is a pinhole-sized leak at a hose clamp or hose which produces a fine stream once the system is up to temp and pressurized. The fluid might hit the exhaust manifold and not leave much trace, or be blown by the radiator fan with little evidence.

I'd suggest getting some fluorescent dye and a UV light, if closer inspection and pressure testing fails to disclose the leak. I assume you aren't getting antifreeze inside at the heater core.
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Art Benstein near Baltimore