dpb writes:

Doug Miller wrote:
In article , "CraigT" wrote:
All right, I've been convinced by you guys to open up the wall.
My prediction of what you will find: absolutely nothing wrong.
I still think the cause is what I said the first time: solvent
exposure, either in the form of overspray from staining the deck, or
whatever was used to clean up the overspray.
Before opening up the wall, at least look at the back side of the
siding you removed. If the problem *was* a heat source inside the
wall, then the damage on the back side should be at least as bad as
the damage on the face. OTOH, if there is no (or only slight)
discoloration visible on the back side, that would be the strongest
possible indication that the source of the damage was on the outside
of the house -- with solvent exposure being IMO the most likely
culprit.

I gotta' go w/ PeteC on this mostly...the OP said the solvent/deck was
done in '06 and this damage didn't show up until within the last two
weeks -- that's pretty long incubation time for a solvent to act.

Looks like discoloration to me.
More than that, looks like the height and shape of a gas grill.
Looks like the heat rose from the grill and went to the left.
It spread out as it rose.
Move that grill up against the fence and see how well the outline
fits. Oh right, too late for that, the siding is gone.

Besides that, there's no need for name calling.

Definitely OP should find out what's going on inside the wall if only
to confirm conclusively it was all a surface damage on the outside...

From what I've seen, I wouldn't.

If we're betting/guessing, I'm still on the side of somebody pushed
the still hot grill over there sometime...

Yep.