behind closed doors

Prepositional phrase

idiomatic

(idiomatic) In private; in one's private life; lacking (normal) public disclosure, in camera. / (idiomatic, sports) Without public spectators.

Japanese Meaning

非公開で（内密に行われることを意味する） / 私的な状況、またはプライベートに行われることを示す / スポーツ文脈では観客を立てずに行うことを意味する場合がある

The board settled the contentious issues behind closed doors to avoid public scrutiny.

取締役会は世間の監視を避けるため、論争の的となっている問題を非公開で解決しました。

Sense（1）

(idiomatic) In private; in one's private life; lacking (normal) public disclosure, in camera.

What you do with your girlfriend behind closed doors is none of my business.

Sense（2）

(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. https://web.archive.org/web/20150611004158/http://in.reuters.com/article/2012/04/20/soccer-world-libya-idINDEE83J07G20120420