This picture shows carpenter ant damage to some 2x4's. Notice the tunnels in the wood, called galleries. The galleries on the bottom piece shows that they can be wide and not deep. On this piece, the ants nested between the sheetrock and the 2x4 stud.
Also notice on the bottom of the bottm piece of wood, there are openings in the wood. These are called windows and allow the carpenter ants to either enter or exit the galleries or remove debris like frass. On exposed beams, these window may be more easily seen than the galleries themselves.