It’s called “Pareidolia.” Because humans are so cooperative, it makes sense for us to be good at recognizing faces. And, more importantly, detecting when someone is looking directly at us and clearly expressing when we are looking at someone else. A predator who lives by not being seen needs a gaze that’s less obvious. In fact, research has shown that our surprisingly white sclera’s, the area that borders the iris, isn’t just an accident, but is a vital piece of human eye morphology that makes it easier for us to ascertain the direction of someone else’s gaze at a glance.
We also have impressive gaze-direction networks inside our brains containing individual neurons that fire when someone is staring directly at us, but that stop firing if that gaze shifts just a degree or two. So, you can tell when you’re being watched, we humans are quite sensitive to it, even those of us with “Scopophobia”: the fear of being stared at. But, to be sure, in order for this to work, the other person’s gaze must be within your line of sight (your field of vision) that is, you can see them. Otherwise, if the stare is coming from outside your line of sight, there is no evidence that people can tell they are being watched.