Stations left over from the cold war that broadcast numbers. To transelate these numbers, you would need a one-time translator issued by the station's goverment directly to their agents. After being used once the code for the station is changed, -as suggested by the name-, and they are burned. Number stations are not avaible to the public, but everyone somehow manages to record it and upload it to youtube or something.
So, am I the only one who thinks Mason listened to a LITTLE too many number stations? Just maybe, a LITTLE?