It showed up for me in a game, too. It seems to be a display bug where a champ will sometimes keep the "mark" on them when the effect isn't. This can lead to a lot of uncertainty cuz unless you see an ally damage them you never know for sure if you can root them. However, I was fortunate enough that when someone did get marked the mark would kind of "overlay" the previous one which made it super bright and obvious. Regardless, still annoying because it seems to happen at random; some champions will get the "ghost mark" while others won't. Hopefully they fix this soon.