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.
(..) very proud of you today, Potter. Bartemius! Not trying to lure Potter into one of the Ministry's summer internships, are we? The last boy who went into the Department of Mysteries never came out! And they say I'm mad. Now, I remember... I remember when I first met you all. Biggest bunch of misfits (..)2005 Harry Potter and the Goblet of Fire
(..) It's you we don't know. You're the juicy news. What quirks lurk beneath those rosy cheeks? What mysteries do the muscles mask? Does courage lie beneath those curls? In short, what makes a champion tick? "Me, Myself & l" want to know. Not to mention my rabid readers. So who's feeling up to sharing? (..)2005 Harry Potter and the Goblet of Fire