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.
You sometimes see arc damage where people have used a plug to wedge bare flex ends in, or the top fuse clip has overheated and melted through the plastic, or a cord grip screw has bitten into the cable and become live, etc. But at the edge of the faceplate it must have been a damaged flex as it's so far away from the plug.
E2A I didn't see the actual arc damage at first, I was looking at the scuff near the left hand earth contact. Now I see it top right, definitely a power arc.
I reckon a plug in low voltage adapter with the cable coming out vertically from the top has suffered a failure around the stress relief gland and there has been a short-lived low voltage high amperage short circuit plasma event in the vicinity of the damage on the face plate. The damage on the faceplate is from plasma splatter.