Numbers Stations. I know very little about them other than what I have read on various web sites. Supposedly they could be linked to espionage, drug running, and other activities that could be considered somewhat nefarious. Some of the broadcast samples I've listened to have included The Lincolnshire Poacher and The Swedish Rhapsody. Both of these samples are included on a 4 CD set called The Conet Project, and there are many, many others. I don't have a copy of the set but I've listened to a fair number of the different samples on line from various web sites. Some of them sound cheezy (after all, they are broadcast on the shortwave bands), and others, like the ones I mentioned above, are rather eerie.
That said, if anyone who operates one of these so-called "non-existent" numbers stations happens to read this and you are looking for something to use as alert music, consider the soundtrack in the video below.