Can you react with Zhonya's Hourglass to save a unit at a different battlefield than where Zhonya's is hidden?

Ruling: Yes, you can react with Zhonya's Hourglass to save a unit at any battlefield, not just the one where it's hidden.

Sequence:

• Zhonya's is played from hidden directly to base (gear cannot be played to battlefields)
• Its effect does not target, so it is not restricted to the battlefield where it was hidden
• The effect applies wherever appropriate

Nuances:

• The restriction on playing cards from hidden only applies to targeting - cards gain "here" as a targeting restriction when played from hidden
• Effects that don't target (like Zhonya's) receive no restriction and apply wherever appropriate
• Gear is always played to base, never to a battlefield, even when played from hidden
• Only units have the "must be played to the battlefield they were played from" restriction