Ah yes, the old spotlight effect. I have this, always thinking people are noticing my every move and judging. I think it's because I've been told several times on another forum that being on the spectrum means you have odd body language that makes people stare, even if you know you act normal and don't stim or anything. I don't think that is a very helpful thing to tell someone with social anxiety. I discussed this with my therapist and he said that body language doesn't give away your identity to strangers in a crowded environment unless it's really obvious. He gave an example, where if a suicidal person was walking through a busy street to get to a bridge to jump from, people wouldn't guess what his intention is by subtle body language even if he was looking sad, and people might only guess if he was making it obvious what he was going to do, enough to draw attention. Even then people probably wouldn't guess accurately, unless they knew him and knew he was suffering mentally, then they might pick up on his body language and guess the worst.
I try to remember those words and it helps a little.