1. taking place secretly or without public knowledge
what happens/people are like behind closed doors
Something that takes place 'behind closed doors' is something that happens in private without witnesses or spectators. This phrase has a negative or accusatory connotation and implies that what was going on 'behind closed doors' is suspect or underhand or of questionable morals. This can be used in social or professional contexts but is most often used to imply suspicion or generate gossip.