Zombie sessions are usually when someone does not logout. It could be because they have not touched their box in forever and someone else stole their license ticket. Otherwise it could be due to signed in on laptop and unplug from network / crash on close, etc.
They don’t count against licenses, they take up a couple hundred bytes each in the db so really are not an issue. They will be cleaned up in 72 hours. (So people can leave their boxes on over the weekend and keep their session).