My guess is that it happened while level sanding the amber toned finish.

"Good sounding guitar but not well built."

Stella!

Is that glue squeeze out below the bridge? I assume there were some structural issues other than just the cosmetic ones?

Also to be clear the only reason why I started this tread was because I was curious to understand this ghosting problem so I won't do it myself some day not to be critical.

"I would never have thought of the possibility of sanding the tint thinner over the stiff braces until this thread."

There seems to be an admonition to new builders to build "light" to get a responsive guitar. Reducing the size of the braces and thinning the scantlings of the back and sides. Like hopping homebrew, It often only shows up as too much in the finished product.
If the braces have formed "hard points" enough to cause "ghosting" from level sanding the finish my guess is the top is rather thin. Some build for today, some build for tomorrow, but the trick is to build something for today that will still be here tomorrow.

Without seeing the guitar in person it is rather difficult to say exactly what is causing the lightness over the braces on this guitar but I have seen something very similar on guitars I have sprayed in lacquer as well as on cars. I think it could be blushing. (moisture trapped in the finish) If the guitar was in a climate controlled shop and moved into a different environment (spray booth) to be sprayed it is possible the builder did not let the guitar acclimate to the new environment. The top wood will acclimate faster than the braces so you will end up getting blushing directly over the braces. If the builder didn't know how to fix the problem he may have just left it and kept spraying more coats thinking it would go away or buff out.

As I said, I have seen this on a couple of guitars I have finished as well as my years spraying cars. Cars will do this quite frequently at every panel that has any structure behind it. The only way to eliminate it is to use a hotter thinner to remove the blushing and/or wait for a less humid day to spray.

Again, without seeing it in person it is hard to tell if it is blushing or sanded through as was suggested above. Blushing sometimes will have a slightly blue cast to it but it can also just appear as a lighter hazy patch. Sanding will usually leave a tell-tale sign of sanding scratches (you may have to look at it under magnification to tell).

That is my take on what it could be.

Bob

Gibson made a lot of guitars like that. As I remember it was late sixties, early seventies and they blamed the new scientific glued that was cured by... some strange process. You could see the location of the braces clear as day.