behind closed doors
Definition from Wiktionary, the free dictionary
Prepositional phrase 
- (idiomatic) In private; in one's private life; lacking (normal) public disclosure.
- What you do with your girlfriend behind closed doors is none of my business.
- (idiomatic, sports) Without public spectators.
- Libya's opening home match in the World Cup qualifiers against Cameroon will be played at a neutral venue and behind closed doors, FIFA said on Friday.