A free app for Android, by Farid Ahmad Ahmadyar.

Hidden Devices Detector is an application to detect and identify any device that is used for surveillance purposes, such as hidden cameras or microphones. It uses a magnetic sensor to detect any magnetic field and determine the direction of the source of the magnetic field. It can also be used to detect other magnetic fields, such as those used for recording and transmitting media, as well as for transmitting wireless communications. 

If you are the victim of surveillance, then you can use this app to detect and identify the device that is used for surveillance purposes. It is a free application that is designed for Android phones.