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.
I found a similar situation in my mom's house (built sometime in the last 30 years).
There was a mystery switch on the dining room wall that didn't do anything. I traced the romex up into the attic, where it turned and went into the cathedral ceiling.
I disconnected it, and was able to trace it part of the way with a toner, but I lost it somewhere in the middle of the ceiling. No signs of any patches in the sheetrock that I could see. I still have no idea where it actually goes.
Anyway, my mom wanted some hanging lights over her dining room table, so I cut in a box high on the wall, cut the romex, and reused the switch to feed a switched outlet to feed the lights. The cables swag in decorative chain over to the wall.
I landed the load end in another box inside the attic for future use if the other end is ever located.
