Upon removing the thermostat cover for an inspection I saw salt..... LOTS of too.
Removing the thermostat from its salt seal, I proceeded to dig out more than a heaping tablespoon of salt from the housing.
Looking at a thermostat with this appearance was convincing enough to consider it "out of action", and RTV'd the cover back in place.
Since there was barely enough wind to move, I was very relieved to not hear the hot alarm squeal after a minute.!!