Yes backdrafting should be checked by a pro. Funny thing though is that usually if the unit is back drafting with the access door attached the door edges would be discolored also. They are not on this unit which leads me to believe that the home owner had to re light the pilot and left the access door off for a while. Maybe until those combustibles next to the unit started to smoke. Vis a vie the fire extinguisher.
You kind of have to look at these things like. Now what could I have done here "IF" I was just another stupid homeowner. Which I am, and things of this same stupid nature, I have done myself, even knowing the consequences.
So what can be expected of the general publi? Anything and everything.
This won't be the first time you see this so set up a boiler plate comment to key into your reports for this one.
Without the stupid homeowner comments for sure. HAHA Wayne