behind closed doors

prepositional phrase3

1. •
   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.
2. •
   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
3. •
   stored in a hidden place.

   • In other research, scientists have found that people who kept cereal out in the open were often about 20 pounds heavier than people who kept cereal behind closed doors. — 2015