Robinson! Yeah, of course it's not a button. The case file clearly says the photo was taken with an automatic aerial mapping camera. It's
what's called an "orthophoto", a standard 9x9-inch mapping photo designed to be used for cartography, photogrammetry, survey, etc. The
camera is looking through an aperture in the belly of the plane and takes pictures automatically by timer as the plane flies a steady
course. The photos have to overlap by a certain amount, etc. It's a standardized process, not a handheld airliner window snapshot that
might get internal reflections. If you read the description and analyst comments here
it's rather obvious. There's no window to reflect buttons from.
The camera was probably mounted something like this
I personally think it was probably some kind of accidental image or reflection or something. It doesn't look like a solid object, more
like some kind of accidental exposure or artifact.