Here's whats happening: you are noticing this girl. You are noticing her behaviors. Other than that, there's no telling what's going on in her head. Often time when you notice people, they notice you as well. This is why a lot of guys think girls are staring at them, but simply they are noticing u look at them and looking back, possibly thinking "why is this guy looking at me". We all do it. It's only a fraction of us that put meaning to it or analyze it beyond a certain point.
To take it even further, what I noticed about a lot of people in general, is that people who are passively thinking about something, in this case, you passively thinking about women, approaching women, dating or we, they tend to notice actions and behaviors of women and for some reason associate that with themselves. While it would make sense to do that, when you have any interaction with a woman or know the woman or talk to them, it doesn't make much sense to do that with random people.
More often than not, when you're unsure of someone's behaviors, its likely that it has nothing to do with you.. and there's no telling what is going on in their head.
Also, Deer in the Headlights look is no "IOI". a possible IOI, can be when a girl who makes eye contact with you, and looks away really quickly or acts instantly nervous in her actions. This can be a sign that she was thinking about you before, and got caught looking at you. Now, another 'POSSIBLE' (but not definite) IOI would be if a girl saw, you made eye contact, and gave you a smile. A blank face is not an IOI. And to add to this as well, a girl smiling at you can also just be a kind gesture.