One of the possible reasons for what happened here was a coiled extension lead plugged in when fully wound, even though nothing plugged in.
I don't buy that, but I guess there aren't that many electrical engineers in the fire service. 
I didn't buy it either, for other reasons explained earlier. But also, if no current flow, no heat. OK, being AC, and being a coil, maybe there is a potential for a few mA, but hardly enough to overheat...
...what there are good at is locating where a fire likely starts, and I think that's why they concentrated on that corner of the
garage rubble. They could prove that it was plugged in, but not that it was switched on (although the switch was on when then found it). Electrical specialist couldn't prove if it was definitely on or off either. They could prove nothing was plugged in the reel.
I must say, it was an impressive sight to watch them locate where and how they thought it started, even if my mind was not focussed at the time.