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.

Thats really interesting. Would be fun to try and replicate this. I was thinking that the Vex Mythoclast might be a key to use inside the portal if you can get in and explore. Like being able to see hidden things only through the scope. Maybe a chest or hidden jumping path? The grimoire says its a tool.

What system are you?