I vaguely remember a story of a man who gets visited by someone he believes to be an old childhood friend. He hadn't seen his friend for a while, so he first ignores the feeling that something is off with his guest's stories (if I remember correctly), but then discovers that the person he's taking to is just pretending to be his childhood friend. He discovers this because he notices that a very small scar (below the eye?) from a wound he himself caused his friend back in the childhood days is not there.
The impostor is from a (secret?) government agency. I believe the man openly confronts the impostor about this, and does not get in a dangerous situation because of this. Rather, they talk about the life of the person the impostor tried to play.
The man lives far from the city, in the woods or mountains if I recall correctly. They talk for some hours, the whole story segment takes place on the deck of the house.
This might have been a short story or part of a book, I don't remember. It is at least 8 years old, bit most likely older.