Can I return a hidden card back to hand?

Yes — but only if the effect can actually target it. A hidden (facedown) card is a legal target for effects that target "a card" or "a facedown card," but you cannot bounce it with an effect that specifically targets "a unit" or "gear," since a facedown card isn't a unit or gear for targeting purposes (FAQ #6679).

When a hidden card changes zones, its owner reveals it (Rule 421.4), so both players see what it is as it returns to hand. Once in hand it's a normal card again — you can play it normally, or re-hide it later by paying the [A] cost again. (Note: if the card is sent to trash because you lost control of the battlefield, that's not a return to hand.)