By saying this, you also say that the spot skill is a skill that only photons and the ability to see affect spot. I do not think so... If you say spot is a skill, then it would need some technique, some rigorous method of deducing what you see. Certainly, every human, by D&D standard see equally well? But what I mean by allowing a person to see through walls, is that the photons do affect the atoms on the other side of the wall, and those atoms again affect the nearer atoms, and so on in a chain reaction. Therefore, from deducing som unknown patterns in those alternating forces, the person with such high ranks in spot, would (in my opinion) be able to get a glimpse of what is happening on the other side...
This was only using photons as explanation. However, there are more delicate things that pass through walls, and you may say from deducing the magnetic (and hence electric) fields that is created in the wall, the thickness may be decided, and even a microscopic difference is needed to change those fields, and may therefore also get a glimpse of what is happening on the other side. This is not pseudoscience (as far as I know) and I think a character that is level 150 may have such attuned senses that such changes are permitted. That being said, I would certainly adjust the difficulty according to the thickness of the wall.
Also, light need not be the source of light in a fantasy world. What led you to think so in the first place? We don't use elements (that being fire, water, air and earth) in our everyday explanation of the nature either, but D&D does. Another thing is that I don't think that we as persons living in a non-fantasy world can comprehend the abilities of a level 150 fantasy character.