Spruce Top Discoloration Under Finish

I have a classical guitar, hand-built in New York about two years ago--I live in Northern Utah. The finish is good and in tact, however, there has recently been a change in the color of the spruce top under the finish in a small area near the top of the upper bout--it has turned slightly gray. The instrument has been well protected from excess heat and humidity has been controlled at about 50 percent. Any ideas on how such a discoloration might occur?

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