Can a player activate hidden cards at a battlefield that is not involved in the current showdown?

Ruling: Hidden cards can be activated during a showdown at a different battlefield, but they must affect the battlefield they are hidden on, so they cannot impact combat at the showdown location.

Nuances:

• The activation itself is legal regardless of where the showdown is occurring
• Hidden cards have an additional requirement that they must affect the battlefield where they are hidden
• This applies to both hidden units and hidden spells