under lock and key

The idiom "behind closed doors" is closely related to the concept of something being "under lock and key." When something is said to be happening "behind closed doors," it means that it is taking place in private, away from public view or scrutiny. This idiom implies a sense of secrecy or confidentiality, similar to the idea of something being locked away and protected. It suggests that there may be hidden or undisclosed information or actions taking place, and that access to such information or actions is restricted to a select group of people.

In a symbolic sense, being "behind closed doors" can also refer to situations or discussions that are kept confidential, such as negotiations, decision-making processes, or sensitive conversations. Just like something that is under lock and key, matters that happen behind closed doors are not easily accessible or known by everyone. This idiom emphasizes the idea of limited access and the need for discretion.

"In a box" is another idiom that is closely related to the notion of something being "under lock and key." When something is said to be "in a box," it means that it is enclosed or sealed within a container of some sort. This implies a level of confinement, isolation, and inaccessibility. Just like something that is under lock and key, an item that is in a box is secured and protected, often from the outside world or from potential damage. This idiom invokes the idea of boundaries and restricted access, emphasizing the need for careful handling and safeguarding.