It's some third-party thing that you put on your browser. I don't remeber the name, but I will look when I get home.
That addon is amazing. No one can hide.
The one caveat with it is that the person has to have actually posted on the forums with that character before for it to show up in the alt-list.
So like, my druid wouldn't show up in the list under my OP-Characters name because I've never used the druid to post before.
Still friggin' great though.