1
general::
In secret, privately. For example, The nominating committee always meets behind closed doors, lest its deliberations become known prematurely. Also see BEHIND THE SCENES.
American Heritage Idioms
2
general::
Phrase(s): behind closed doors
in secret; away from observers, reporters, or intruders, usually in a closed room. • They held the meeting behind closed doors, as the law allowed. • Every important issue was decided behind closed doors.
McGrawhill's American Idioms And Phrasal Verbs