When emptying a fleet of all its ships there will sometimes be a white box fleet left behind.
It can’t be moved (shows message that ships in the fleet can’t go to the destination).
The fleet can defend a planet but loses immediately and the battle can only be started with auto-deploy.
The fleet can be eaten by drag dropping another fleet on top of it.
The fleet info screen can’t be opened so ships cannot be built into it or dragged into it.