Mystery

"Obviously you feel it's something we should look at more carefully."
"A mystery is only a mystery as long as it remains uninvestigated, sir."
"I love a mystery, Data, but this one will have to to wait..."
Jean-Luc Picard and Data, 2364 ("Lonely Among Us")
"This is what you do for fun?"
"It's a mystery. Who was this man? Who killed him? Where's the money he was talking about? It's a mystery. Now, we have to go search for clues."
"Ah, and that's fun?"
Guinan and Jean-Luc Picard, regarding Picard's enjoyment in the Dixon Hill series, 2367 ("Clues")

A mystery was any unsolved secret, such as a disappearance or murder.

In 2000, Alexandria Books had a "mystery" section. (VOY: "11:59")

The disappearances of Judge Crater and Amelia Earhart, along with the fate of the Terra Nova colony remained mysteries as of 2151. The fate of the Terra Nova colony was solved by the crew of Enterprise in that year. (ENT: "Terra Nova") Amelia Earhart was found by the crew of USS Voyager in 2371. (VOY: "The 37's")