Can a hidden card remain on an empty battlefield, and if so, can it trigger when a unit enters?

Ruling: Hidden cards cannot remain on an empty battlefield. When you lose control of a battlefield (it becomes empty), any hidden cards are removed at the next cleanup.

Sequence:

• When a battlefield becomes empty, hidden cards remain until the next cleanup
• At the next cleanup, hidden cards on empty battlefields are put into the trash
• If another player empties a battlefield on their turn, the hidden will be gone by your turn

Nuances:

• The owner of a hidden card can activate it before the battlefield is cleared, as long as it wasn't hidden that same turn
• You haven't lost control of the battlefield yet if you're the one clearing it, so you can still use the hidden before cleanup