Solar Power
This house has a bump out on the rear that happens to face a particular direction. During the months of May and July the sun's angle is such that it reflects off a bedroom window and onto the vinyl siding. You can see the more direct and closer reflection and where it is more oblique. I see this a lot really. And what can be done? I bet this happened the first year or two of the house's existence. Replacing the siding will result in the same effect you see here! I have seen this same effect on a house next door to where this same reflection comes! That heat can travel!