The iMog is an electrical device, based on an iPod. It was initially created and passed down by one of the Greeter's that met Number Nine and Sandor upon arriving on Earth. Sandor later modified it to look like an IPod to avoid drawing attention to it.
The device uses the essence of a Mogadorian (by collecting their death ash) to detect when Mogadorians are nearby. However it is prone to interference as Mogadorian genetic material has a habit of rapidly decaying (hence bursting into ash when killed). The iMog screen has a white dot in the centre, representing the user and when Mogadorians are nearby they are represented by red dots.
Upon Nines capture by the Mogadorians, the Soldier mused at its function before destroying it. It is stated that Sandor had another, more heavy-duty version of the iMog which he used to search for any Mogs while he drove around Chicago, it is unknown what happened to it.