Chris Lewis wrote:
According to dpb :
I don't see it as "around" the railing, I see it as to the right and
concentrated just about at the height of the railing post. I could
easily see a heat source centered at that point producing the pattern.

If it were a heat source behind or in the wall, I wouldn't expect
to see the bottom of each vinyl siding piece (where it's bumped out
from the wall) being as badly damaged as that compared to the top.
The air gap has to account for something.

[If it was on the inside of the house, like a toaster flameout,
there should be much more obvious heat damage on the inside walls -
an exterior wall doesn't transmit heat very well, especially in
cold climates with decent insulation.]

Still, it'd be a good idea to at least remove the covers and
electrical devices from the boxes and take a really close look
inside and see if there's any signs of damage to the wiring.
On the inside too, if there's any electricals (eg: switches) there.

In any event, he's going to want to get the damaged siding off so
it can be repaired, and that should make the cause a lot more
obvious and/or some of the sheathing can be cut out to check
the wiring more directly.

Conjecture is easy but useless.

The siding will have to be replaced anyway and simply pulling it off (or
even one piece) will show whether there's heat damage to the
siding/homasote/whatever and be conclusive that it is/is not from an
internal heat or external heat source. That's well worth knowing
immediately imo, just on the chance that it is/was.

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