How come that we "feel" it when someone's watching us
To quote the University of Sidney site:


  People often think that other people are staring at them even when
  they aren't research led by the University of Sydney has found.
  
  When in doubt, the human brain is more likely to tell its owner that
  they're under the gaze of another person, researchers from the
  University of Sydney and The Vision Centre reveal in a recent article
  in Current Biology.


Also, from the scientific paper:


  We presented participants with synthetic faces viewed under high and
  low levels of uncertainty and manipulated the faces by adding noise to
  the eyes. Then, we asked the participants to judge relative gaze
  directions. We found that all participants systematically perceived
  the noisy gaze as being directed more toward them. This suggests that
  the adult nervous system internally represents a prior for gaze and
  highlights the importance of experience in developing our
  interpretation of another’s gaze.


So you, and I, overestimate how often people is staring at us.

For details, see: http://sydney.edu.au/news/84.html?newsstoryid=11335 and https://www.cell.com/current-biology/retrieve/pii/S0960982213003321