Secret radio universe
I also have a AM/FM/shortwave with a digital tuner and I discovered a whole band of stations just below the AM band. They appear to be legit, too. Many are local communities that disseminate public service messages, safety information, weather, community activities, etc.
Then there are the "number stations". I haven't heard any for a while, but they usually involved a deadpan human voice in English or Spanish repeating long sequences of 5-digit number groups. (Usually on the AM frequencies.)
Espionage? Military? Drug cartels? UFO aliens?