This is an outlet at my in-laws’ house which was built in the 1960s. I’m not sure if this is just some paint that was put on an outlet and has worn off over time or if this is some residual evidence of sparking/minor electrical fire. The walls are wood panels painted white, so it seems to me someone might’ve just thought they could paint the outlets as well (there are some parts of this basement that don’t have the wood painted and the outlets/switches are dark brown, which might be what I’m seeing). The outlet seems to work but I don’t feel totally comfortable plugging something more expensive like my laptop in, even though I suppose the charger would be more likely to die than the actual computer. The house has a fairly new electrical panel with breakers that seem nice, but it was also built initially as a duplex (with a later addition making it useable as a triplex) and each part has its own old sub-panel that still uses twist-in fuses. I’m still not sure if whoever put in the new panel said the rest of the wiring/outlets was fine or my father-in-law just decided upgrading the rest was too expensive.
From that photo alone, that looks like someone didn't want to deal with changing the outlets when they painted the walls and that's just paint warn off of a dark brown receptacle from many years of plugs being inserted.
As one who has done many dumb things in my life, evidence of small electrical fires would be whispy smokey gray possibly with some meltiness.
Side note, autocorrect tried to make "dumb" be "fun" and I couldn't feel more seen right now.