What happens to a hidden Zhonya's Hourglass when a unit leaves the battlefield?

A hidden Zhonya's Hourglass does not get revealed automatically when a unit leaves a battlefield. You must play it when you have Priority; it then takes effect and returns to base. If the card is still hidden when you lose control of the battlefield, it is lost and does nothing. If you play it and then one of your units anywhere would die, Zhonya's Hourglass is destroyed instead.