Is there a Unload unit Event ?

Hello! I was searching for a Event : Unload unit ,for many hours and cant find one. I can see A unit loaded another unit but cant find the unload, and I need that event for my map. Without that event I wont be able to know if a unit like a zep got any unit in it or not. If anyone know a work around or anything that can help me, plz reply. Thank you.