I'm pretty sure that's the meaning GP was going for. The only other meaning I can think of would be "serving in the military", which (in the US at least) would

I'm pretty sure that's the meaning GP was going for. The only other meaning I can think of would be "serving in the military", which (in the US at least) would be "in the service" and anyway members of the military also remove their hats indoors except in certain specific circumstances.