When the guy who masturbated at the crime site is being interrogated in the basement, a random character walks into the room. He's a handyman. He repairs something in the background and leave in the middle of the scene.
What could it mean? Does it have any significance? It feels awfully odd and out of place.
I thought the scene was there to show how incompetent they are for discussing details of the murder while a civilian worker is there. Police investigators should never discuss sensitive information around people who shouldn't hear it, yet they're having a full dialogue while the guy is checking out the furnace.
I don't think there were supposed to be any clues in the movie as to who the killer is. The whole point of the story is that the investigators believe someone is guilty in order to fulfill their own personal narrative. Even the detective from Seoul wants to believe so strongly that the young ex-military guy is the killer, despite when the DNA evidence says he's not. The real killer is probably someone who they had no clues about at all.