Behind closed doors
Definition 1 of 2
Pro Tip 1/3Confidential Context
Used when referring to sensitive matters needing privacy. 
The HR issues are often resolved behind closed doors, to protect privacy.
Pro Tip 2/3Exclusive Meetings
Indicates exclusive meetings, not open to general public or media. 
The executive session was held behind closed doors, with no press admitted.
Pro Tip 3/3Discretion
Highlights the importance of discretion or secrecy in certain situations. 
Witness protection strategies are discussed behind closed doors for safety reasons.