Can a hidden Zhonya's Hourglass be played from one battlefield to affect a unit at a different battlefield?

Ruling: Yes, Zhonya's Hourglass can be played while hidden at one battlefield and affect units at another battlefield. When a gear is played hidden at a battlefield, it is placed in your base instead, and Zhonya's effect is global.

Sequence:

• When you play a gear that is hidden at a battlefield, it is placed in your base
• Gears cannot stay at a battlefield on their own
• The gear's effect then applies according to its normal rules

Nuances:

• Hidden cards that make choices must select/choose from the battlefield they're hidden at (like Foxfire)
• Zhonya's Hourglass doesn't require choosing, so it works globally regardless of where it's played from
• This follows the same logic as playing it as a normal gear - you wouldn't get a choice of battlefield in either case