Same thing happened to my team the other day, Venus side, unmarked, free to explore. It has never happened to me either! And I don't know how we weren't marked, we killed him on second teleport. I noticed lights in there that I hadn't seen before, like when traversing the "scenic rout" from the first chest to Templar, red lights that lead the way. We too eventually died from our exploration attempts. I was convinced something special was happening, but haven't identified what. Needless to say it looked wonderful in there with no marking mucking up my screen.