u/Comfortable-Mud8813

I’m in the middle of plastering my walls after wallpaper removal so I can paint instead. Im preparing to sand the plaster smooth.

Because I don’t want dust to get all over the house I have taped up a tarp on the ceiling to contain the dust.

There’s no light behind the tarp so I brought in a flashlight, but all I could find was a blacklight flashlight. When I turned it on, I saw this.

What happens during construction of a house that creates these perfectly round circles?