Probably got in while house vacant. Ants got in and died from previous treatment.
The ants are getting in, probably around the foundation. Buy about 10 tubes of caulk and caulk every nook and cranny around entire perimeter of house, and any joints in siding up to about the first 2-3 feet.
The ants are getting in somewhere, and rather than poison them, don't give them a visitor pass, and lock them out.