Do hidden cards remain on the battlefield if it is no longer under my control (i.e., if I no longer have units there)?

Ruling: No, hidden cards do not remain on battlefields where you no longer have units. Hidden cards are removed from battlefields that do not have a unit controlled by the same player present and are placed in their owner's trash.

Sequence:

• Check if a hidden card is on a battlefield
• Check if that battlefield has a unit controlled by the same player who controls the hidden card
• If no unit is present, remove the hidden card and place it in the owner's trash