Can a player react with a hidden card when their unit is being moved off a battlefield by an opponent's spell/unit/effect?

Ruling: Yes, a player can react with a hidden card when an opponent's spell/unit/effect that moves their unit goes on the chain.

Sequence:

• Opponent's spell/unit/effect goes on the chain
• Player can react with their hidden card
• The hidden card is restricted in what it can target (same restrictions as other hidden cards)

Nuances:

• The hidden card must not have been hidden that same turn
• There must be a legal target for the hidden card
• Hidden cards cannot react to permanents and Add effects