Level 68 Monolith Run, I think I was on Echo 16. It was a Lost Catacombs map and a Shrine Cleanse quest.
Activated the shrine and after the first two waves nothing else spawned. I cleared everything in all adjoining rooms. Finally figured out there was a creature spawned along a south wall that you could see behind some crates and it never agro-ed. Was just sitting there and I only found it because I started walking along facing southern walls and swinging to see if there was something behind them.