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.
(..) Where do you go all the time? Around. Peter Parker. - Maybe you'll tell me who she is. - Who's that? - This mystery girl Harry's been dating. - Dad. - When do I get to meet her? - Dad! Well, sorry. Harry hasn't mentioned her. (..)2002 Spider-Man