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The mark does not come off when rubbed or scrubbed lightly. It doesn’t streak or smudge. It is to the side and the bottom of the outlet cut-out.

The outlet is in the sub-grade lower level of a split level house. It is located on an external wall nearish a fireplace that has not been used in years.

The external walls here have moisture issues, the sheetrock near the ground being saturated when using a moisture meter.

Is this moisture? Soot/dust accumulating due to electrostatic attraction? Mold? Electrical issue? Something else?enter image description here

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Your guesses are on the right track: soot/dust sticking to the wall caused by temperature or moisture. I'll take a guess based on what I see. It's not electrostatic or electrical or mold.

My guess is you have forced-air heating or A/C. Given the locations of the air intakes and supply vents, and which interior doors are open or closed, there can be negative pressure in some rooms. Air is being sucked in from the wall cavities by the negative pressure in this room. The air flow in this case, and perhaps its moisture, is depositing dust in the pattern you see.

How to confirm this?

  • Loosen that screw half a turn, wait 3 or so years, and see if the pattern changes. :)
  • Add a supply vent in this room to balance the pressure or add return vents elsewhere. These kinds of things will affect overall temperature balance in your home so they need some planning.
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  • dust or soot from burning candles Feb 3 at 18:32
  • There’s a vent directly above this outlet, so I bet this is right. Thanks!
    – Stew
    Feb 3 at 19:30
  • Is it a return vent above? A supply vent nearby would counteract (but not disprove) my answer.
    – jay613
    Feb 3 at 19:49
  • 2
    They make foam insulation pads that go between the outlet and the faceplate -- mainly used to make a home more airtight from an insulation standpoint, but maybe it would help you here by sealing up that outlet box a little bit. amazon.com/Duck-Sealers-Variety-Decorative-283333/dp/B0040JH21W
    – mikem
    Feb 5 at 6:37

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