The thing with ants... they don't nest inside structures, they nest in the ground (although I have seen them nest in potted plants outside the house).
Because of this, the only sure way to rid a structure of them is to treat the perimeter of the home. Perhaps carpenter ants are different, but I've found this to be true of the common Argentine ant here.
When I got this place in '97 I had yearly problems with ants inside the bedroom, of all places. They were coming in from the garden (the bedroom is at the back of the home), and bivouacking inside the ventilation openings in the foundation. I tried a LOT of different types of control, but the only product that has really worked has been the Amdro perimeter "Ant Block" granule product. Since I used it about five years ago the ants have disappeared all over. They used to even nest inside the pump housing for the fish pond - a lot of them, and they would keep on coming back despite various sprays and baits.. A little Amdro in there and they've disappeared from there as well.
Your ant trails may vary. But the lesson I learned is that baiting the ants inside the house is a very poor substitute for treating the entire perimeter of the home. And forget arsenic based baits. The Argentine ants seem ignore it, or they are immune to it.
