hidden knowledge

A hidden camera or spy camera is a camera used to photograph or record subjects, often people, without their knowledge. The camera may be considered "hidden" because it is not visible to the subject being filmed, or is disguised as another object. Hidden cameras are often considered a surveillance tool.
The term "hidden camera" is commonly used when subjects are unaware that they are being recorded, usually lacking their knowledge and consent; the term "spy camera" is generally used when the subject would object to being recorded if they were aware of the camera's presence. In contrast, the phrase "security camera" refers to cameras that are visible and/or are accompanied by a warning notice of their presence, so the subject is aware of the camera's presence and knows they are being filmed.
The use of hidden cameras raises personal privacy issues. There may be legal aspects to consider, depending on the jurisdiction in which they are used.

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