If Zhonya's is hidden on a battlefield and an enemy unit is moved to a different battlefield, can that Zhonya's be used to protect the unit?

Ruling: Yes, Zhonya's can protect a unit anywhere on the board when played from hidden, regardless of which battlefield it was hidden on.

Sequence:

• Zhonya's is played from hidden
• Because gear cannot be on battlefields, Zhonya's is immediately recalled to base
• From base, it can protect a unit anywhere on the board

Nuances:

• Zhonya's is unique among hidden cards because it is gear, and gear cannot remain on battlefields