Who is the strange figure in WVW?

For a week now, I keep seeing a strange figure in WVW. I think it is mounted. It appears in a puff of smoke, with sound effects, and then is consumed in a second puff of smoke and is gone. It doesn't happen often, so I've failed to get a screen shot.

My best guess is it is a visual artifact of seeing an invisible GM as I move into the draw distance. I can imagine that ANET would put some real flair into their appearance/disappearance.

Nevermind, I think I found it. It's a player. Experimentation shows that I can see if it is behind me, but I can't see it if it is in front of me. Probably a hacker. I'll give up now, because I've never found a way to report anything that is paid attention to.

Well today I saw someone on a mount that was halfway underground if that makes any difference lol.

It's Bootts trolling you on his minstrel deadeye build

Slightly more information: I could only see him if I was mounted and my character was facing away from him. He answered once when I started talking to him, then he logged out.