A: “In Play” always means, for all cards: laying on the table with it’s face showing – not in the draw pile, or the discard pile, and not in someone’s hand, not face down, not hiding under some other card.

Keep in mind that, for Creepers, you’re not allowed to hold them in your hand, so if they’re not in play, they’re either in the draw pile or the discard pile (or they may be covered by some other card, like a Keeper which effectively nullifies them rendering them as if they were not in play. The Elder Sign in Cthulhu Fluxx can cover (nullify) one Creeper in this way, as can Batman Cuffs.