An interesting case. Taking the homeowner at face value, nothing providing a load. The condition of the front of the box tends to support that. There is no duplex outlet plug-in evident, you would likely have a bare two or three wire male plug w/its insulation burned-off coming out the front of the outlet. Nothing resembles that here.
If I was the AHJ for investigation of this, I would have clipped off the box wires back about a foot from the box, beyond any romex charring, and used a prybar to remove the box from the stud. I would have looked at the point of attachment under the wallbox to the stud, to see what type of deterioration there was between the box and the stud.
I agree that it appears the combustion began in the box. I may have missed a description of the type of box used, metal or plastic?
Back at the station, I would have removed the outlet from the box trying to keep-intact as-possible the front plate, the outlet, and the box. Lots of pictures from all angles would be done. Careful disassembly would have preserved the integrity of the components involved.
I also would have removed the adjoining duplex outlet on the same circuit, and taken that for purposes of comparison. I would have looked at the branch circuit breaker, taking plenty of pictures, and possibly using a sharpie to initial and date the circuit breaker, if it was going to be left in the branch panel.
The likelihood is that the duplex outlets were brought at the same time from the same place, and may have been in a 'contractor's pack,' say 10 duplex outlets packaged together. It's possible that a defect in one during a production run may be found in others manufactured at the same time. A disassembly of the adjoining component may (or may-not) reveal some issue of interest.
The usage that duplex outlets are subjected-to could also cause component failure. How-many of us have had to replace a duplex outlet due to lateral stress on an indwelling-plug fracturing the ground plastic or the hot or neutral slot? Rough usage can cause wear or failed components which may be seen by careful disassembly and evaluation.
Generally, electrical components like duplex outlets work or they don't, out of the box. If they don't it's one that slipped past whatever quality control methods are used by the manufacturer. That may be a standard random sampling of a timeframe of production, or it may be an 'every unit tested' method, I think probably the latter. If it makes it out of the factory, and is properly-installed in the end-user's application, it will probably give long service for the expected lifespan of the device. Premature failure is likely due to faulty installation not to accepted industry standards, or rough usage after installation, without replacement after experiencing a component failure of some-sort.
I am a career firefighter/paramedic, I am a licensed fire inspector, a fire service instructor, a plans examiner, and all those are current. My career began over 40 years ago, I'm now retired from the fire service but I keep my certifications and licenses up-to-date by college classes, in-service instruction, professional journals, and professional associations.
