Possible beetle feces
Unidentified frass. Sonora California. 12 October 2017. Collected from basement, by Josh Esposito.
Dr. Andrew Sutherland thinks, but is far from certain, that this is feces from a developing beetle larvae, given the varying size. They are very similar to frass collected from a drawer that had powder post beetles. See https://foothillpest.com/technical-pages/frass-images/beetle-frass-from-drawer/