Since the gallery is more oval than round, it could be from a flat-headed borer or round-headed borer. The larvae make a gallery or channel in the wood of dead trees, and are packed with frass (partially-digested wood) from digested wood. The frass is the powdery substance you found. The tree is cut into lumber and the larvae continue to grow inside. Except for the old house borer, none of these beetles reinfest wood in structures after they have emerged. Consult a professional to look at the situation.