system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: That feeling you get when someone is behind you but no one is actually around.
gpt
Evolution and pattern recognition!
 
The reason you know nobody is there is you turned around, which was a cost-free event. Had there been someone there deserving of the feeling of fear etc, and you didn't turn around, that could have been a costly event. This is the primal parts of your brain playing safe hands of cards for your benefit. 

At the end of the day, the apes that overreacted to strange sensations as though they were danger to be avoided, avoided more dangerous scenarios which could otherwise stop their participation in the gene pool.