According to Pete C. :

Look again, closely and with an open mind. You're the one fixated on an
external source. I've looked at that possibility and have not found any
likely external source that would cause the damage seen in the photo.
The position of the damage extending only about 12" in front of the
railing and continuing behind the railing points strongly against pretty
much any likely external heat source, including a grill, hibachi, jar of
iced tea, decorative sun catcher, etc. The fact that the Tyvek was
heated to the point that holes formed in it while the siding only sagged
also points away from an external source.

I'm still holding out an open mind on it. I don't think it's even
that conclusive. Nor am I sure that I fully accept the OP's knowledge
and recollection of every possibility.

Secondly, Tyvek != vinyl. The vinyl may show less damage than the
tyvek even if there was an external heat source. The tyvek may have
a lower melt point than vinyl, will likely shrink (causing tension)
when heated, and it's so thin to begin with, it'll blow holes much
more quickly than the vinyl would.

Given the information we have to date, I still think he needs
to pull some sheathing off. The tyvek melt pattern is too suggestive
of a hot wire for my comfort.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.