PSA: if you see two people walking "in file" on the street close together, chances are the person in front doesn't know that someone is walking that closely behind them. if you dont see two people walking side-by-side, its safe to assume they dont know each other.

I have experienced this personally and also have seen this happen with others. Someone will just be walking, most of the time a man, and another man will walk very closely behind. The person walking behind knows how to walk undetectable. Some people might see it as walking together but its a complete stranger following someone that closely. Its creepy, and it might be a way men hook up with other men, but it should be assumed the two do not know each other, and the one in the front does not know they are being followed.