Some woods will darken just because they are exposed to oxygen. An osage bow will darken even if it is kept away from light. My collection of Native American flutes which are made from various woods (cedar, osage, spruce, Alaskan cedar, hickory, ect.) have all darkened with exposure to the air alone.
Wood just does that, even if it is under glass. I have an all osage longbow with glass back and belly that has been in a bow sock and in a box for years. It was so bright when it was new that it was blinding. Now it is a deep honey color and a real beauty.