Can a hidden unit at one battlefield be revealed as a reaction when an opponent targets a unit at a different battlefield?

Ruling: You can reveal a hidden unit as a reaction even when your opponent targets a unit at a different battlefield. However, the revealed unit's reaction ability can only target cards at the battlefield where it is revealed.

Nuances:

• The hidden unit can be revealed in response to actions at other battlefields
• Any targeting restrictions (such as "here") on the revealed unit's ability still apply to its own battlefield