Sometimes they are just traveling, the nest might be outside like a wood pile, RR tie walls, etc. Check aroound and it could be a ways away from your house.
http://www.terro.com/
Sorry I missed your post earlier. About a month ago I was plagued with two separate ant infestations. Weather they were seeking water, food, or escape from water (heavy rains here) I didn't care. I wanted them gone. The guy who sold me this stuff at the local hardware said they had sold it for over 25 years and never had a return. This stuff works. Place a little of the liquid bait on a piece of cardboard and they take it to the colony. Once it spreads around, the colony collapses; end of ant problem. I will never use anything else.