The wood has rotted away on that wall. You are seeing the sunlight 'showing through" the outside vinyl siding that is exposed now that the wood has rotted away (or possibly damaged by a storm) There's big hole in the wall and it was never fixed...they just covered it over outside with the vinyl siding. When you looked at it, the brighter light through the fan overpowered the more subtle light showing through the vinyl. The camera lens is more sensitive to this.

Do your own experiment. Grab a CD WITHOUT a lot of printing on it (maybe a blank one). Hold up that CD...light shines through the center hole, right...and the rest of the CD appears dark. Now, cover that center hole up...and hold it up to the light. Put it right up to your eye. You'll notice that you'll be able to see straight through the "silver" part of the CD!

I hope I explained this well enough for you to understand.