The feeling of being observed, that we are not alone, is something that everyone has experienced at some point. Scientists have conducted in-depth tests and came to the following conclusions:

1. Error in interpretation
Surely you have experienced a situation where you looked back and the person returned your gaze. We can only guess who looked at whom first, but we won't get an answer. Why our brains synchronize with others in a particular moment is something we cannot explain. In fact, we cannot rationally explain this because we do not know why it would be necessary. Hmm...

2. Subtle clues wake us up
Our brains often function as they did millions of years ago, when every noise, movement in the surroundings, change in light, shadow movement would wake us up. If this happens to you, you can be reassured because a change did happen, but how the brain perceived it, we still don't know.

3. We feel like someone is watching us, but we don't see them
This is the most controversial case, for which we know that it involves nonverbal communication that originated from times when (early) humans did not speak but expressed love and even anger this way. The ability to nonverbally sense emotions is still characteristic of ancient peoples (for example: Australian Aborigines).

The issue with all these cases is that scientists acknowledge that the brain perceives what's happening in our environment, we just can't explain it.

Probably because we don't understand how the brain can "see" the world without eyes.

In the past, people who had such visions were often attributed with psychological problems, but today we know that we don't fully understand the workings of the brain. In between are also such and similar paranormal cases.

Hmm...