I think I have figured out what's going on. You can see in this pic, there's space around the pots on this desk. There's a piece of paper in there toward the window with a bunch of succulent leaves. (Waiting for them to make roots before putting into pots.)
Last night, I noticed there's a bunch of dirt on the paper, falling from the hanging pot above it, probably because there's a worm in the pot. I think I've been hearing the bits of dirt falling onto the paper.
