If your ants aren't pharaoh, you may want to check here . It's small size enables it to get into almost anything. Because its food preference ranges from things like syrups, jellies, pastries and grease to toothpaste it has the faculty of appearing all of a sudden in various places in the house. These ants are usually seen trailing along established paths in kitchens and bathrooms. The pharaoh ant's nature of being scattered about, living in walls and other well protected areas makes them difficult to exterminate.
Control almost has to be with a bait.
From what I've read you shouldn't spray them. Spraying can break one colony into several, making difficult control even more difficult. Spraying their trails may cause the to avoid the bait too. There is a widely touted homemade bait made from boric acid powder and some foodstuff they like. Many think mint apple jelly is best.