That's an interesting one. I can't believe it's some kind of flash-over arc from unplugging a max 13A load, unless the socket is wired up from the wrong side of a 11kV transformer
Could it be someone messing about with a piezo gas igniter?
Or nothing to do with electricity: just local failure of the plating on a rubbish scrap-iron back-plate with included impurities? Is it always seen on the edge, where the metal is cut and the plating probably thinner on the right angle? Plating failure can easily be started by contamination with alkaline plasters, grout, etc on installation.
Although I agree it does look like an electrical arc, like it's been touched by a live core, by some electrical maintenance hero working live, because he couldn't be arsed to go downstairs to switch off. But that could never happen!
Edit: Is there an induction hob in front? I wonder if it's possible for eddy currents to arc from a cooking vessel to an earthed metal plate in near contact?
Edit 2: The more I look at it I think it's chemical corrosion. Maybe some exotic electro-chemical process, but nothing to do with our sort of mains ac.