[Sterling 6.7]

You mentioned that this is located in the bedroom. It is possible that there may have been a thread or fuzz or hair or something that could have laid across the prongs as the cord was pulled away from the wall. As the cord was pulled out, the material laid across the prongs causing the short, the short disintegrated the material and left the burnt mark.

You mentioned that the outlet was relatively new and sees little use. The photos would make it seem that the smoke is only on the surface and the outlet has not actually melted. If this is true, then an overload is less likely to have been the case.

Probably worth replacing the outlet for cheep peace of mind. I would toss the cord as well.

Side note - when I was working on construction jobs in years past, I would always get on the jobs where the framers and sheetrockers would crank their radios up. I was tempted to take a single strand from a piece of #12 THHN and make a single wrap around the prongs on their cord and leave it lay next to the spider box. I could never get myself to do it, but I have always wondered how they might have reacted. Now everything is battery and earbuds, opportunity lost I guess.