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.
(..) To the end! We're all in. Daft buggers. We finish this. Truly, you are a wizard! The mystery is in the black powder. Surely the Lord will grant us victory. You'll be by the gate to cut off reinforcements. John, you sit on this wall to protect our escape. (..)1991 Robin Hood: Prince of Thieves