Hidden cameras can be wired or wireless with regard to the video signal. They also need power which they can get via being plugged into wall power, batteries (which have to be replaced when dead or by a recharging implementation like a sonar panel, wind turbine etc.), and inductive leeching (according to stories, but I have never seen one work this way). Each variant of the hidden device has it's own strengths and weaknesses and requires a different approach to detect. Companies that do this typically have an arsenal of equipment at their disposal. Some devices look for power-related anomalies, some for RF, Some use image processing to reveal the reflection of the lens of the hidden camera, etc. If a camera is sending the signal through the air, it will be easy to locate with nearly any RF receiver. If the signal is sent on a wire, and the wire is properly shielded, your average, off the shelf detector will probly not see it. A cheap tracer probe just might!
Check frys.com
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