We've noticed this fine brown dust seems to be coming down from the ceiling around our masonry fireplace (fireplace has not been used so it's not debris from burning anything). Does anyone have any thoughts about what it could be?

Additional context-

- It looks almost black in the photo, but when we used a wet paper towel to clean it, it's definitely brown and smears when wet (maybe animal or insect droppings?)

- This accumulated over the course of 2-3 weeks (had been wiped up before)

- We initially though it might just be from storms and slight shifting of our large masonry fireplace, but it happens regardless of weather/wind/etc

- We had all of the flashing replaced around the chimey thinking that might fix the issue, but no luck

Curious if others have seen anything like this.