A few situations I remember:
Buying an eyeliner at the self checkout grocerie store. The woman at the service podium stared at me as I walked out. I just smiled to her and she did this look like she was caught off guard because I smiled at her - she didn't really know how to react. She read me, but I didn't let it bother me.
When walking by a smoking area (lone female standing outside smoking with nothing better to do but watch people). From about 20 feet away she watched without a smile or expression, but kept focused on me. I felt like she was judging me (not sure about being read because her expression would probably have changed) and after a while she directed her eyes elsewhere.
Again a smoker, outside a hair salon. I guessed she worked there. I walked right by her and she turned to watch me. I smiled as I walked by, but she did not return a smile. I guessed she read me so I put a little extra wiggle in my walk to rub it in, but she may have just been critical of my clothing or hair.
So either way, I think the stare is an evaluation and it is the moment just before you are read or not. If you get read by an unfriendly person, I think this where they stare you down to say you are not fooling anyone and by a friendly person, probably a big smile or laugh instead. Usually big eyes and opened mouth come at the point most people read you.
