I’m noticing three patches of black dust by the skirting boards in my kitchen. I had laminate put down a couple of years ago but have only seen this recently. I removed the trim to check and it looks almost like it’s blowing up from the floor but the floor is concrete with underlay. It also comes back in half an hour when I clean it. Anyone come across this before? It’s not insect waste (looked online and it’s the wrong colour and texture) and feels a bit like fine coal dust. Confused and worried!