When you play a hidden Zhonya's Hourglass as a reaction, does it get recalled to your base or stay on the battlefield, and can it only affect units on the battlefield where it was hidden?

Ruling: When Zhonya's Hourglass is played from hidden as a reaction, it only affects the battlefield where it was originally hidden, regardless of where the Gear physically ends up. The exact timing of when it gets recalled is unclear, but it likely gets recalled to your base after it resolves (during the next Cleanup), while still maintaining its restriction to only affect that specific battlefield.

Nuances:

• Zhonya's creates a replacement effect that must be in play before a unit dies, so it's always played before the death condition is met
• A Cleanup runs after each item on the chain resolves, which would normally recall Gear back to base
• If you lose control of the battlefield where Zhonya's was hidden, it would be recalled to your base rather than remaining on an opponent-controlled battlefield
• The Gear never "fizzles" or goes to trash - it stays on the board somewhere
• This interaction is acknowledged as unclear and will be clarified in future rules updates
• "Hiding" a card is different from "playing" it; flipping from hidden equals playing