If a player passes before revealing a mission does the unresolved/un-attempted mission get discarded or does it go back into the players hand?

Back to your hand.

RR page 6 "A player may choose not to reveal a mission that he assigned a leader to. At the end of the Command Phase, any leaders that are still on mission cards are returned to their leader pool, and the mission cards are returned to the player’s hand without being revealed."