To add to Kyle’s response, an undercoated airplane does not guarantee corrosion protection. The two airplanes I’ve performed the most corrosion repair on were both coated yet the material still corroded. It corroded from the inside out, which may have happened to the airplane in question as well. Problems like that can be quite concerning because you may not see anything more than a slight bulge in the material for a while but the core is dust. Eventually it will pop through and make itself easily seen.