What happens to a hidden card on a battlefield when the only unit on that battlefield is moved away by an opponent's spell (Charm)?

Ruling: If the hidden card is not revealed as a reaction and it becomes the only card remaining on the battlefield, it gets removed during cleanup.

Nuances:

• Hidden cards can still be played as reactions even when there's no showdown on their battlefield, as long as you could normally play a reaction at that timing
• Hidden cards can only target units/cards at the battlefield where they are hidden
• If a hidden card is played by paying its full cost (not as a hidden card), it can target units at any battlefield or base