I have seen in game where Zed's ultimate on his Deathsworn skin will sit above a champion and not go away. I do not know if this is my client or a bug in the game but I have been noticing it a lot when I spectate a Zed with that skin or have a Zed with that skin in my game.
I'd be willing to bet you're looking at the extra AD indicator. When ulting and killing a champion will give zed extra AD they get an indicator above their head.
Expanding on what Dorans Shuriken said, Zed gets an icon, usually a shuriken, but differs between skins, above an enemies head if they will give more AD to his ult passive if he can kill them with his ult on them.
