locked room; sealed room
A room that is locked or sealed from the inside, often used in mystery contexts (密室殺人). Also used more generally for any room with no apparent entry or exit.
Solve the mystery of the locked-room murder.
The windows and doors were all shut; it was a completely sealed room.
secret room; hidden chamber
A room that is kept hidden or secret, not necessarily locked. Often implies a concealed space for private meetings or storage.
There seems to be a secret room in this mansion.
The elevator stopped, and the room became sealed off.
He was trapped in a locked room and called for help.
The important decision was made behind closed doors.
Participants must escape from the locked room within the time limit.