I've even noticed that as I've become more self-confident and take up more energetic space that I've started getting called "sir." It's always by men and usually they aren't looking directly at me, but I think they just register tall and confident as masculine. It used to really piss me off. I'm wearing an effing flowered mini-skirt and have big boobs and you think I'm a sir? Now I just (mostly) accept that it's a part of the subconscious belief that only men are supposed to "read" that way. A few people have commented that maybe they think I'm trans, but I don't think that's it, since they are invariably extremely embarrassed once they realize what they've said.