That kinda looks like the 'blushing' that occurs when spraying nitro in high humidity. I.E. The affected area sits within the finish. I'm unaware if the same thing happens with polyurethane (in whatever application method).......but it's very odd that it wasn't there during polishing. If you look across the surface at a very low angle, can you see any step down into those spots? That might give you a better idea of whether it is sanding related.