An apparatus and method to automatically determine the location of unknown media devices is disclosed. An example apparatus includes a media device detector to detect an unknown media device identified in monitoring data collected by an audience measurement device at a location determined to have an “on” status, a media transmission detector to detect a media transmission associated with a device address and an association storer to store an association of the device address, the unknown media device and the location. A probability determiner determines a probability that the unknown media device is located at the location.