Are you sure the scale didn't come off? It looks indented in the first picture, like it's actually just the skin from under the scale. Like Bogertophis said, just the right spot for rubbing. The skin itself if that's what's going on, looks healthy. So I wouldn't worry.
Otherwise, some animals do just change color with age. I wouldn't be surprised to see a few darker or lighter color scales depending on the species.
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