When the crew removed the dry wall on MasterBath/Day08, Tracy noticed that they revealed a portion of the wall with exterior shingles, facing into the room. This wasn't immediately apparent, because the front and back of the shingles are all the same color. However, the overlap pattern of the shingles clearly indicate that we were looking at an exterior wall. Near the ceiling, I could discern a sloping roofline.
So how did an external section of wall find its way into our bathroom? I think I figured it out.
Originally, the house did not have a master bedroom.
At some point, somebody added a window bench. The interior exterior wall is the side of the window bench.
Finally, somebody built the master bedroom.
The shingles are not painted, hinting that the house's current greennishness is a recent development.