David.Fournier

I believe these replies are out of date. Sentinels have been revamped. They now have a single "vulnerability" power that charges, replacing all of what's described below.

OK, someone on the help channel just replied. Posting the answer for future folks (or if I play this again and forget!) They are in the prison cells. You have to bash down all the doors. I was certain that was not the answer, because the prison cells all have view slots, and if you look in, they look empty. Since they looked empty and bashing the doors takes forever, I hadn't tried it.

I haven't been able to complete this either. I can actually only get the mission as an Ouroboros flashback (must have out-leveled the contact?) so I can't force complete it either. I am just totally stuck and can't complete the arc. This is in fact a Council cave map. I've tried the "inch by inch" approach many times. Plus, all the rooms are very familiar from 100 other Council caves and I'm aware of what the "hidden" stairways look like and where they're found. I strongly suspect there's something I have to click on that's not obvious and not glowing. As The Swamper mentioned, there is a huge door in the room where you "turn yourself over" and on the map there is clearly a way to go behind it, but no obvious way to open it.