There are many types of spy camera (also called hidden camera) in the market. They are either very small (not visible to the naked eye up to certain distance, normally few meters) or disguise itself as something else such as a smoke detector, motion sensor or even a clock.
I would never install a spy camera for home user for whatever reasons. So, don’t bother calling me for the services.
However, hidden cameras may be necessary for commercial use. It may not be possible to cover every inches of your retail store with CCTV coverage or the cost would be extremely high. An experienced shoplifter would know which corner to avoid looking at the angle of your camera. The solution: Spy camera to catch those with negative intentions.
Catch them in order to teach them a lesson so that others would not follow.