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Possibly, before finishing, someone used a filler on the wood (on a "defect"?) and applied the (stain?) finish over it. The filler didn't stain, didn't accept the stain and/or it was the wrong or mis-matched color filler.
If this is so, then lightly sand the area and color-in the white marks with (as best you can find) a marking pen, marks-a-lot, artist's paint, whatever, then apply a top coat finish.