hinter verschlossenen Türen — behind closed doors. At the cabinet retreat at Meseberg in March 2023, the news agency dpa wrote: Ausgeruhte Gespräche hinter verschlossener Tür scheinen nötig. Rested talks behind a closed door seem necessary.
hinter den Kulissen — behind the scenes. The same report recalled the early days of the coalition, als strittige Themen in erster Linie hinter den Kulissen ausgefochten wurden, when disputed topics were fought out mainly behind the scenes.
in Klausur gehen — to go into closed session. A Klausurtagung is a meeting closed to the public. The word comes from Latin clausura, closure. In German, Klausur also names the part of a monastery that outsiders may not enter.