The Hide in Shadows skill could have helped getting the scales, if you had a skill point to spare.
In my game a couple ghosts wandered into that room after the doors opened; don't know if that was the reason, but Amdusias didn't attack me when I returned (he did when I first entered). Did you get any ghosts in there? If not maybe Amdusius just wanders out of range, and doesn't see you? At least one time I noticed he was circling around to the far left as you enter.
