basically what unUnder said. as long as you see there's black trails on the floors where there are barrels/boxes try moving them to where the black trails lead new pathways can be found that way as well. I can't really rmb which room exactly but from the entrance of the ruin, turn left and don't head to the place where there's red blue green icons on the floor. eventually after moving some barrels around you'll find a room where it only leads to a door and this door will lead you to the rock thingy that will give you access to the thief. ofc needa get the rusty keys too but sorry i can't rmb the locations for the keys