I know everyone is doing Infusion runs, and the mission too, but what's the bonus? I haven't seen any unexplained items or bosses lurking about, and know one there seems to know the Bonus.
I did the bonsu on my necro months ago and my warrior about a week or so ago.
Basically instead of killing markis in the top fort you go south until you pass all the white mantle and mursaat. (VERY long path, longer than actual mission) You see an NPC complain about corpses everywhere and ideas being stolen by the Infiltrator. Then you chase after the infiltrator and kill him.
Bonus is actually pretty much near the start of the mission, and takes like 2 minutes, but noone EVER does it because you have to be infused to do it and Infusion is done much later in the mission and noone wants to backtrack and noone wants to re-do the mission after infused.
The biggest problems remain that Henchies are not infused, there is no tangible reward for completing this bonus, and getting an 8 person group to play the game with no chance of an Ecto or Shard is nigh-on impossible.