There maybe possible Spoliers aswell, whatever they are

Hey guys, i've just hit the Watery Depths part of chapter 3-5 and may have found some bug/glitch just wondering if anyone has noticed or ran into this.
To clarify i'll list what i've done in this place so far
Gone in - Caddoc opens the "wall that sounds hollow" -retrieved the key
Proceeded into the first spiral room, listened to the Ancient ones riddle
E'lara shot the boat, lighting "wet logs", turned the first wheel.
Shot the brazier, lit up a side passage
Gone into the room with the mechanical bridge, killed a few skeles
Pulled the lever so Caddoc could cross, burned the tapestries - found 2 secret rooms
turned the wheel 3 times, water level dropped
gone back into the room with the book/hollow wall, killed an elite skele
gone down the stairs where the water was, i assume this is the part about "wet logs lighting the way"
gone into the tunnel, spider bursts out attempts to munch someone, when freed retreats into a hole in the ceiling
looted the stuff down there, went into the spider tunnel, attempted to shoot the claws in the ceiling (green circle) nothing happens
procceded into the tunnel, ran to the top and found the exit, objective still incomplete
whenever i go near the entrance Caddoc mentions that something may still need finding
EDIT: after killing the Elite skeleton, theres a red flash that appears every 2 seconds or such, cant seem to locate the source of it
i've ran through this place on both chars now, for a good 2 hours reloaded 3-4 times and still can't find what i'm missing. Is there some super secret to this place? or is this down to a bug or glitch?