I was doing the omnigul strike and when you are fighting her, there is a boomer knight on a platform just inside the main boss fight room. if you jump up there you'll see a symbol glowing on the wall that is exactly the same as symbol that is on that thing that is just inside the timed door in the raid. I think that thing is a timer and if you get through the door at a certain time you get another chest. there are symbols all over the omnigul fight area and I wouldn't be surprised if it had something to do with another chest