If you suspect the nest is behind a wall or in a hollow door, find a way to get an insecticide into the nest. Use any cracks and crevices for application. Treating just outside surfaces will keep ants from view because they are repelled by sprays. However, surface sprays do not kill the queen and will not solve the problem. Done correctly, a single treatment should eliminate the nest.
Dusts are best for treating voids.
You may need a plastic squeeze bottle or turkey baster to blow the dust into the hole. Small, light applications are more effective than heavy ones. Dusts will not hurt insulation. Liquid or aerosol sprays for ant and roach control can also be used. These will work well if the nest is exposed.