behind closed doors

behind closed doors (in a secret meeting, only for insiders, in a room secured from outside intrusion) — за закрытыми дверями, за закрытыми дверьми; тайно

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...New figures published on Thursday offer a chilling insight into the violence and abuse that goes on behind closed doors. According to the data, two million people in England and Wales suffered sexual assault, violence, threats or abuse at the hands of a partner or family member last year.

Related
[behind the scenes]
[keep under one's hat]
[sweep sth under the carpet]
[under wraps]
behind sb's back