Update....I just got off the phone with my trusty motorcycle/scooter mechanic and he said internal condensation is most likely the cause. He said that because the cavity in the final drive is only partially filled with oil, the empty portion of the final drive actually "sweats" inside. My garage does get damp when the outside temperature drastically changes, so it is feasible that this could be the problem. The only part that doesn't make sense is that none of my other six scooters, including my other two Kymco 250's, have had this issue.
Thanks again to all who responded. Your suggestions that moisture is definitely the culprit was spot on.