If you can't find it, at least become keen observers of where the ants go and hire a pest control company. Once the nest has been found, it needs to be removed and the queen ant has to be killed. Reputable pest control service may use chemical or nonchemical means to remove the nest and treat your home over the course of a year to stop subsequent infestations. Most firms that remove ant nests recommend a series of follow-up visits in one package price. Once the source of the problem is pinpointed, repair the leaky pipe, spongy wood, leaky roof or other material that is providing the perfect breeding grounds for the ants.
Ant colonies can re-establish if the same conditions remain.
It's also a good idea to make your house less hospitable to ants. Remove any old wood piles or at least don't store wood in your house or near the house. Trim trees so branches don't touch the house or roof. Ants will frequently gain access to the house by crawling up old, rotting trees onto your roof. Check the foundation for holes too.