I'm not convinced its a sensor of any kind. I think it's a spot on the button with no (or less) black paint on it. Either a manufacturing defect where the button was attached to a spine or holder, or it's for the center pin of the button mechanics. It's far to small to be a fingerprint scanner and there's no mystery scanners built in to Apple hardware.
Either way, if it's visible under normal circumstances, then it's reasonable to return it and request a unit without a cosmetic defect. If it's only visible in the most unusual of lighting situations - and the unit is otherwise pristine - I personally would probably ignore it.