I had noticed this wall sconce in one of the images I discarded from my last visit and couldn't believe I hadn't actually focused on it. When we returned, it turned out that what I had seen was a reflection of the sconce, not a sconce sitting where it appeared to be. This is still a reflection, but at least I know where it is now!

My apologies, I ended up switching pictures here. Same scene, just a bit more of the mirror and the wall.

It was a real challenge to keep me out of these mirror pictures.