Why do ants suddenly appear in the house?

Ants can suddenly appear when they find food or water, or when changing conditions disturb their usual feeding or nesting areas. Foraging workers may lay scent trails that bring more ants to the same spot. A busy trail can develop quickly without a new colony having formed inside your home. (UC IPM: Ant identification and management; Texas A&M: Ant behaviour)

Check where the trail enters and what it leads to, including spills, pet food, bins or moisture around sinks. Store food in sealed containers, fix leaks and wipe away trails with soapy water. Removing the attraction and checking entry points is more useful than repeatedly wiping up ants without finding the source. (UC IPM: Ant identification and management)