http://www.bbc.com/future/story/20170801-the-ghostly-radio-station-that-no-one-claims-to-run
There's been a fair amount of attention given to these sorts of mysterious radio stations in fictional media. There was The Numbers Station a few years ago with John Cusack (a lousy movie). Something similar was covered in an episode of Covert Affairs (not a terrible show).
here's some more
http://secondnexus.com/technology-and-innovation/russian-spy-radio/?viewall=true