The sawdust from nowhere

In the picture is a slipper with bits or particles of an unknown material. The slipper was positioned at the edge of the roof in the backyard so it is logical to look up and see where it was coming from. This is the 3rd time that I have seen those particles in a period of 3 weeks. Of course, I had cleaned the first one and also the second one. Now it is time to investigate again. But there is nothing unusual on the roof, the wooden ceiling is normal.

The particles are quite soft like cork. It may be edible because there are some ants that are starting to investigate the particles. I am posting this article to get some ideas form any one here that may have encountered this kind of phenomenon. Those particles appeared 3 times here and it seems that they were coming from nowhere.