The first thing I usually notice is whether or not the person will meet my eyes or not.
I will meet peoples eyes. I tend to not look people in the eyes for reasons most people never suspect. The first is, I can generally tell a lot more about a person instantly then I ever want to know by looking in their eyes. Seriously, I know EXACTLY what type of person they are. If its someone I know I will have limited time with, yeah I generally (while not making great effort) simply dont do it. Because I can do this, I generally assume other people can do it to me even though thats rarely, if ever is the case. I assume they can see me recognizing them for who they are and that will reflect on how they treat me.
Its not an issue of shyness or anything like it. I just honestly dont like doing it. When I am in crowded areas, I turn my hearing off as well. I have gotten very good at it over the years. As an introvert and observer, I pick up a whole lot of information that I dont necessary want to. It can be very draining energy wise because of what I now assume my mind is doing. Processing it all.
Anyway, I have to wonder what other people think of it sometimes.

Am I hiding something etc...
The other thing is, I assume someones going to figure out how intelligent I am. I wonder if anyone remembers the Highlander movie where when Connors family figures out hes immortal and banishes him because of it. Its something they dont understand and even though these are the people that are supposed to have loved him through his entire life, they banish him because he is something they dont understand. Sometimes I feel like if people know how much I really know, theyll not understand and be scared of it.
Now, I know this sounds like I am tooting my own horn but... Im not talking just about knowledge. If a person is extremely perceptive of things around them, it might seem like they have some form of ESP or mind reading ability. People are scared of that crap still even though we live in the 21st century.