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.
(..) on his own initiative. Me, a friend of his boss. On his own initiative. I'm asking you again. I have no idea, Nicolas. It's a mystery. A mystery indeed, Dominique. I've spoken to you, as would say President Chirac, "from the heart". I hope so, but not in Chirac's terms. (..)2016 Me Before You