Can you activate a hidden card from a battlefield that is not currently being attacked?

Ruling: Yes, you can activate a hidden card from a battlefield that is not currently in combat. However, hidden cards have the added text "here" for targets, so any targeting effects can only target things in the hidden card's own battlefield.

Sequence:

• You can play the hidden card during a showdown at the other battlefield
• The card activates normally
• Any targeting restrictions apply to the battlefield where the hidden card was placed

Nuances:

• Hidden units must be played to their own battlefield
• Cards without targets (like draw effects) work fully from the non-combat battlefield
• Cards with targets can still be played but can only target units/things in their own battlefield
• Most hidden cards can include the unit that exists with the hidden as a legal target, so be careful not to accidentally penalize yourself since legal targets cannot be ignored