One scout finds a crumb. An hour later there's a highway of them across your counter.
Ants follow invisible pheromone trails laid by scouts — once a trail leads inside, the whole colony follows. They slip through cracks a fraction of an inch wide, foraging for food and water.
Sprays kill the ones you see, but the colony just sends more foragers down the same trail.
Foraging trails stop at the barrier. The Nano Film band around the foundation is a surface ants can't climb — the scout never makes it inside, so the trail never gets built.