Hardwood floors: What could leave spots of bare wood?
 

We are finding bare spots on our hardwood floor, (i.e. apparently missing the polyurethane sealant). We have found spots in the middle of the floor (away from windows) in a few locations. The wood looks bare. The edges of the spots missing the polyurethane are crisp. I do not believe this is water damage. It doesn't appear to have "bubbled up" characteristic of the water damage that I've seen.

Was a nasty spill cleaned up recently as that might do it.either that or it was applied to thin.