If I move a unit from a battlefield where I have a hidden card, what happens to the hidden card?

Ruling: When you move a unit from a battlefield, the hidden card gets trashed. You cannot activate the hidden card in reaction to moving the unit.

Sequence:

• Unit moves (e.g., Merchant)
• The move trigger is added to the chain as "pending"
• Cleanup begins
• Control of the battlefield is lost and the hidden card is trashed (step 4 of cleanup)
• The move trigger is "finalized" onto the chain (step 8 of cleanup)

Nuances:

• During a showdown, control cannot be lost, so hidden cards can still be activated even if your unit is killed
• Spells like Gust create a chain with reaction windows where hidden cards can be played before the unit moves
• A standard move action does not create a chain or reaction window
• This applies to any move (standard move or moved by an effect) - the cleanup rules apply to all moves