What caused this? Termites? Carpenter ants?

[Bob H.]

I am in the process of buying a house in Fort Collins, CO. The house was built in 2000. I found this debris next to the frame of the back door to the garage. The other side of the frame, while equally in need of paint, did not have the debris. 

I probed the wood underneath the frame with a car key. It felt pretty solid, but later when my home inspector (not a pest inspector) probed with a pocket knife, he was able to dig out a little bit of wood.

I looked for mud tunnels on the outside of the foundation and in accessible areas of the garage and basement. I did not find any.

What do you think caused this? Termites? Carpenter ants?