There's no 'mystery' (to/as to/about) why we're here. = It's no 'mystery' why we're here. [=we know why we are here]
His success is something of a 'mystery'. = His success is a bit of a 'mystery'.
We'll reveal the identity of today's 'mystery' guest after these commercials.
(..) They caught a gander at the display in the back and they figured they's gonna run out and solve the great Deadwood mystery. And how'd they get that idea? I wrote 'em a map. Out to the old farm road. I figured, what the hell? It wouldn't do no harm. (..)2003 House of 1000 Corpses