I worked in a 30's house in San Jose years ago. There was a mystery switch on the wall of the living room that we had no idea where it went. During the rewire, the owner wanted some sconce lights on each side of the fireplace at a 6' height and about a foot or so away from the chimney chase............
..............You guessed it, 2 round K&T pancake boxes plastered over in those exact locations from old sconce lights. It was actual plaster that filled these boxes. The wires were capped with porcelain wire nuts, so maybe they were live the whole time if the switch was flipped on.
Same house was also insulated in the 50's with that blow-in paper stuff and old newspapers were used to line the plaster lathe between the ceiling joists. All the papers dated from 1952. Very interesting reading.
