I was called out to troubleshoot and fix the lights just outside the front door. They had shorted out, causing the circuit breaker to trip. Previously, another electrician had been called out; he was able to disconnect the shorted circuit, but couldn't find the fault.
I opened the switch box, and found the disconnected wire. It was part of a three conductor wire (black red white). I inspected the outside boxes where the outside lights mounted, and found only two conductor wires (black white). Bad news: clearly there was a splice somewhere, and I couldn't find it. It sure looked like someone had covered it up with drywall, a violation of NEC 314.29 which requires boxes to be accessible without removing any part of the building.
I got out my wire tracer, and it seemed like the wire ran along the trim of the door. I talked to the homeowner, who said the floor plan had been changed sometime after the house was built, but before they had bought it. She located the original drawings, which indicated the door had originally been hinged on the right side. This was enough to give me a theory. The original switch had been on the left side of the door. All I had to do, was poke a hole in the wall on the other side, find the buried box, fix my wiring, and remount it at the surface of the wall with a blank cover. I got permission to proceed, made some exploratory holes, and found ....... nothing.
I thought the wire had been run around the door, so I tried removing the trim from the door frame, and found not one, but two buried splices: without boxes, a second National Electric Code violation: NEC 300.15 which requires a box to be installed at each splice point.
At one of the splices, I found my problem. There was a mystery wire that was shorted. I disconnected it and everything started working.
I believe this wire may lead outside to a post light which had been removed at some point and had become shorted out (by a shovel perhaps). In any case the homeowner didn't know of any devices that weren't working, and I left this wire disconnected.
Next I had to fix up these splices so the next electrician wouldn't have to spend all day doing a simple repair. Fortunately, the spliced wires were long enough for me to locate new boxes near the splices. One just above the door, and another one in the door frame on the other side of the wall.
I tested again, installed cover plates, replaced the door trim, patched my holes and cleaned up my mess.