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Comments to #3
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“No idea what this is. It’s powdery and reappears quickly (within a couple days) after being vacuumed up.”
Comments to this post suggest different answers, from pieces of insulation falling out of the door frame, to ants and termites damaging the house. In fact, the presence of termites may be a plausible answer, since these creatures leave powdery mud formations when they start destroying a house.