It's based off of a real life phenomenon called The Third Man. Definition: The Third Man factor or Third Man syndrome refers to the reported situations where an unseen presence such as a "spirit" provides comfort or support during traumatic experiences. It also goes on to provide examples like a world renowned mountain climber who's team and him said they saw a third man with them. After he told his story it caused a great amount of people to come out with they're own very similar stories. Very intriguing eh? Cheers.