What can Pack of Wonders return to my hand?

Pack of Wonders can choose a friendly gear, a friendly unit, or a friendly facedown card hidden at a battlefield. It cannot return a card with the [Hidden] keyword from anywhere else, such as a player’s trash. To make that clearer, it’s receiving minor errata to say “facedown card” rather than “[Hidden] card.” [E]: Return another friendly gear, unit, or facedown card to its owner's hand.

Related: Pack of Wonders