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.
(..) He was a great man. Would you like some port? - It's really very fine. - No. I can't help you, Lara. This clock truly is ... mystery. - Hello? - Lara? Hello, it's Mr Wilson. Yes... Well, you see... I had a second thought about that clock. (..)2001 Lara Croft: Tomb Raider