Can a hidden card at Battlefield A be triggered during a showdown at Battlefield B if it doesn't target anything or spawn units (like Consult the Past to draw cards)?

Ruling: You can play hidden cards any time you can play a reaction, regardless of which battlefield they are hidden at.

Nuances:

• If a hidden card requires a target, that target must be at the battlefield where the card is hidden
• This targeting restriction applies to units that trigger after they resolve (like Blastcone or Teemo) and cards that don't specify targets (like Zhonyas)
• You can play a hidden card as a reaction outside of a showdown at its battlefield, even if it has no valid targets (e.g., using Blastcone Fae to maintain battlefield control when your unit is killed)