In a Detective Drama, any time a piece of dialogue comes along which is off-the-cuff, not followed up and unrelated to everything, you can tell it's going to be very important. If a suspect turns up late and says "Sorry I'm late, my car was stolen yesterday", the alleged car theft will be significant. If the detective remarks that the suspect has a nice keychain and the suspect says "Yeah, it's from my old fraternity", the insignia on the keychain will turn up later to reveal that the suspect and victim were in college together. Basically this happens whenever the writer can't find a neat way of dropping an important clue into an existing conversation.