I know that I cannot be the first and only person to see this. But could it possibly be that the Sandbenders of the Si Wong Desert are the Last Nation of Airbenders. For one thing Sandbending techniques resemble Water and Airbending more than they do standard Earthbending. In additon, some of the sandbenters are covered from head to toe, easily hiding any airbender tatooes. They are distrustful and dismissive of foreigners which would be reasonable given air nomad persecution. Their Sand Sailers are moved with a tornado of sand, and sand is just as easily moved with air bending as earth bending. Lastly, when the sand benders stole and sold Appa, it could also have been to Appa would not have attracted attention or increased suspicion about poosible air nomad descendants living in the area.