A hidden camera or spy camera is a still or video camera used to film people without their knowledge. The camera is “hidden” because it is either not visible to the subject being filmed, or is disguised as another object. https://www.youtube.com/watch?v=QZ5GGJiBWvM
Hidden cameras can be built into commonly used objects such as television sets, smoke detectors, clock radios, motion detectors, ball caps, plants, and mobile phones.
The use of hidden cameras raises personal privacy issues and there may be legal aspects to consider, depending on the jurisdiction in which use takes place. A hidden camera can be contrasted with CCTV, which is visible and which sometimes is accompanied with a warning notice of its presence.