As a firefighter for 33 years and a building official for 28 years, I think we can all agree that is was residual electrons spilling out from unused receptacles that could have been the problem. That is why it is important to place the safety caps in any plug not being used. Seriously, electrical gets blamed on a lot of fires that probably was not the true cause. In my area, we document our findings and let the insurance company determine if they want additional forensic investigation. As a victim of a house fire, I think that is was the result of a home stereo circuit board overheating but have no additional evidence. It was reported as electrical on the fire report. No one wants to state that the cause of a fire was undetermined, but sometimes that is just the way it has to be. If you point to a certain cause, then the lawyers start getting involved.
Fire at a friends house a few years ago. I was called right away but wasn't allowed in until after Fire marshal investigator had been there. Did not meet this investigator either.
Fire was in vicinity of a fireplace (wood burning fireplace). Most fire damage was confined to immediate fireplace vicinity and what was above that vicinity.
Fire investigator determined fire started immediately adjacent to the fireplace - was an outlet there that served a TV, feed through cable went on to fireplace blower unit. Investigator never gave an exact cause, just that it started at that outlet location.
Using this information when I was allowed in, wires were bare no insulation or sheath in that vicinity, but that is understandable. They were still connected to what was left of the receptacle, metallic components only. Things had been disturbed enough I wasn't going to say exactly what might of happened either. Did not notice any remains of an outlet box, but do understand a plastic box possibly was completely consumed in the fire or I just never found any remains - they had been disturbed by firefighters and possibly by fire investigator.
They were using fire place that night, they used it pretty much all the time during heating season. My best guess - glowing connection at terminations on that receptacle with the fireplace fan being only load contributing to it, is the most likely cause of that fire. I told them that was my best guess but I wasn't able to confirm it. This blower would only be a 100 VA load, if that.