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.
(..) Do you want to say something? I... ...was... ...wondering if you're still in the Village. You're such a mystery. Peter. What? Happy birthday. I'm seeing somebody now. You mean, like a boyfriend? (..)2004 Spider-Man 2