I think there's a tendency for NTs to assume everyone they meet is NT until proven otherwise (much like straight people tend to assume everyone is straight, I think? I am not sure if that is a good analogy, but it's the best I have), so yeah - I think it's easy to get passed as NT even if you look autistic or do autistic things in front of people. Plus people coming up with alternate explanations (I know people tend to come up with their own reasons for why I sometimes get stuck on a word or phrase and repeat it, and it's never autistic - even though it was the first thing I noticed about myself that made me think it) as Anbuend says.
I mean, I am not sure what people think about me, but I've caught myself stimming in various ways - rocking, finger-flicking, staring at lights, clicking my fingernails in my ear, kind of shaking one or both hands (not really flapping, I have these long floppy sleeves on this thing I wear, and I like the feeling of them on my hands), smiling inappropriately (talking to my niece about how she was arrested today? I couldn't stop smiling... I hate that). I tune out of conversations the moment they shift to things I'm not interested in, I talk at length about the things I am interested in, I have to stop and decide how to react to strong emotion, like I'm on a kind of delay, when most people would immediately know what to do, I tend toward very little clothing variety and I tend to eat the same food over and over again. I wear sunglasses everywhere but in the house (and sometimes in the house). I occasionally (although not too often for the past couple of years) try to sound like my favorite characters from television, movies, and video games.
And while I feel like I have become more obviously autistic in the past month and a half, most of the above is stuff I've done for a long time. I figure if anyone in this house doesn't know, they don't want to know, or they don't know what to look for, or they have alternative explanations they're comfortable with (possibly not flattering to me).