Symptoms
- two lights on the circuit continued to work
- the dead part of the circuit had a short (0,8ohms)

I removed all wall plates and pulled forward the switches and sockets and probed for 120V on the wires using a pen type device, and checked for loose marrettes

I finally used a ms6818 and scanned all the wiring and found 120 behind the outside bathroom wall.

I disconnected the GFI and switch in the bathroom and the short still existed.

I then pulled out the socket on the outside of the bathroom wall and could see nothing, the MS6818 detected nothing.

I then used the pen device again and if I positioned it carefully I detected 120V and the first 1 1/2 inches of the live coming into the box for the socket.

Upon further inspection I found that hidden by the insulation the live wire had arced against the box (probably cut or slit insulation) after 20 or 40 years and I could see sputters of copper on the box and both ends of the wire had melted into smooth lumps.

All of this had been hidden by the insulation which still connected the two end. All of this occurred in a bend in the wire in the very back bottom left of the box.

After the wire had melted open the live end had sprung up and no longer touched the box and the end going to socket and the rest of the circuit sprung against the box cause the short to neutral/ground that I measured on the rest of the circuit.

What a pain - all repaired and working now.

The 120 volt probing devices and the MS6818 are much less sensitive when probing inside a metal box.

In fact, the through the wall probing stops working even as you approach a box. Got to love physics.