Recently we have been getting small black ants coming into the house. Through summer we have been relatively free from these ants in the house, they have tended to stay outdoors. We have seen many ant lines in the yard, and across walls, following paths - the ants obviously looking for food to feed the nest. We have had some very heavy recent rains, and as is the usual, the ants started looking for dryer accommodation. They are sneaky insects, using little seen areas to track along walls, windows and anywhere else they think they can go in the search for food. Any scraps that fall on the floor are fair game for them, particularly if they fall, or are kicked, into less conspicuous places.
When the tracks are discovered outdoors, application of powder is easy, tends to stop them fairly quickly. Indoors the use of a "sticky trap" works but not always so quickly. Despite all good efforts though, these little critters seem to be able to outwit us, finding new places to nest, making new Queens to build the colony again.