Well I'll include the two things I mentioned in the last post...
Maybe also people standing still, since it's harder to watch someone while you're moving around.
We can ignore the blind and any aliens whose species is know for poor eyesight.
I want to say people who are alone, but I have no reason to think who ever's watching me isn't with a group of people. So never mind that.
Also, perhaps, people who are clearly doing something, reading, looking in a shop window, etc. To try disguise the fact they're watching someone. I'm thinking spy movies where they pretend to read the newspaper, but are really watching their target.
Nothing else comes to mind.