Vilemaw Vision?

So I was playing a bot game on TT and our team was attacking Vilemaw. The bot team had a Karthus who had just re-spawned, and he seemed to know perfectly when to use his ult to kill 2 of us while we were still attacking Vilemaw. Needless to say we didn't end up killing the beast There weren't any wards or anything that would have granted the bot team vision of the top area, and I had always assumed that attacking Vilemaw was similar to attacking Baron in the sense that the other team wouldn't know you were actually there unless they physically saw you there (or they just assumed you were since no one had vision of any of your team).

So I'm just wondering if there is a different vision structure on TT that I'm not aware of, or if Karthus somehow ran into a bug in being able to ult us. Yes, he could ave just chosen to randomly use his ult, but I've never seen a Karthus bot do that unless he knew he might be able to kill someone.