Can a hidden Hourglass (Zhonya's) be activated to save a Snapvine at base when it's targeted by Falling Star at a different battlefield?

Ruling: No, Zhonya's Hourglass ability is restricted to the battlefield it was hidden at, and cannot be used to save units at the base from a different battlefield.

Nuances:

• This ruling is based on stated intention from Riot developers, not just the written rules
• There is debate about whether the hidden restriction applies to playing the card versus its effect, with some arguing the effect should be unrestricted similar to Tideturner
• The comprehensive rules create an apparent contradiction by stating gears can only be played at base AND that hidden cards may only choose targets at their battlefield