Do you notice how people happen to turn and look at you' when you're looking at them?...like they sense your eyes on them. And the other way round

It's because we see a lot more detail than we actually focus on, the brain decides what we need to see and what we don't but it's all done in the background nit consciously (it's why a lot of optical illusions like the disappearing dots work). So basically you pick up on clues that someone is watching you it could be their reflection in an object someone else's reaction to them looking etc whatever the clue is it's too subtle for you to notice consciously but your subconscious notices it and if it decides it's a possible threat it gets to your conscious mind as that "I'm being watched" or "something is wrong" feeling that makes you check your surroundings.