The phrase "mysterious past" refers to someone's history or background that is hidden or unknown, creating an air of curiosity or intrigue. Full definition
With her trusty parasol in hand, Alexis heads northward by dirigible, determined to solve the mystery... and maybe find out more about her husband's mysterious past as well. (goodreads.com)
During the course of the struggle, Batman learns the truth about his own mysterious past, and the role played by the Joker in shaping his life when he was a boy. (dvdmg.com)
And who knows --- they may share some studio and game secrets, reveal undiscovered Easter Eggs, and shed some light on mysterious past projects! (blog.us.playstation.com)