system
Given the following post, respond as if you're commenting on Reddit:
human
What causes that "being watched" feeling?

I can understand the likely evolutionary reason for it (your ancestors would have been more likely to survive if they were aware of predators), and I recognize it's often a false alarm, but what explains the times when you actually *do* notice being watched by someone who isn't even in your field of vision? What sense would alert you to that? Have there been any good studies on the subject, considering it's difficult to properly test?
gpt
Basically, when you are in a situation where your brain believes you may be threatened (night time, alone, in a dangerous area) your brain makes you think you are being watched in order to increase your alertness and responsiveness to your environment.

It's a context sensitive response. Sometimes people do happen to be watching you.

Semi-related, your brain is very good at distinguishing between when someone is looking directly at you and when they are looking past you (like directly over your shoulder). This isn't quite what you're asking since if involves actually seeing the other individual, but you might find it interesting.