A flood in our basement revealed that we had a big nest of carpenter ants. We got professionals in to get rid of them. Carpenter ants are big, black and shiney. I mean they are very big. They are very aggressive with big mandibles. One interesting trait about carpenter ants is that you will never see them expose themselves in groups. They only leave the nest to gather food. They do this by following establish scent trails. And they space themselves apart. This way you never really think that you've got an infestation. You'll only ever see an occasional ant.
Carpenter ants do not eat wood for food. They will eat/chew wood to make pathways for their nest. Our little guys had the main nest under the floor. It was about 6' x 10'. They had chewed up huge amounts of 2x4 flooring beams. Branching out from the nest were tributaries to all four corners of the level. They ate through amazing amouts of wood. Luckily for us they did not eat through the wall studs. But the whole floor had to be redone. And this nest, as big as it was, went un-noticed for around 10 years. Why? Because they never travel in groups.
My advice, try and capture on of the little buggers and get it looked at by an outfit that knows ants.