The Lincolnshire Poacher
During the Cold War from the 1950s to the 1980s, shortwave radio enthusiasts across the world began noticing weird broadcasts that would often start with music or the sound of beeps, which would be followed by even more strangeness -- the voice of a woman counting in German, for example, or a child's voice reciting letters from the alphabet in English. Listeners gave them amusing names, such as "Nancy Adam Susan," "The Swedish Rhapsody" or "The Gong Station." Listeners assumed they were signals for secret messages to spies [source: Sorrel-Dejerine].
One of the strangest was "The Lincolnshire Poacher," which used a snippet of an English folk song of that name. After about 10 minutes of music, a female voice with an English accent would read what appeared to be a coded message. According to Dutch cryptology historian Dirk Rijmenants, the Lincolnshire Poacher appeared during the 1970s and aired daily until 2008, when it mysteriously disappeared. The common assumption is that it was some sort of communication between British intelligence and agents in the field, but there's never been any official confirmation, so it still qualifies as unidentified.