An ant's scent trail usually becomes first noticed inside a home through a cracked area in a wall or floor. It may also be first visible from a baseboard along the floor. Other entry points might be unsealed holes to the exterior, including plumbing entry points. Usually the trail continues in a fairly straight line towards a food source. Often the food source is an area of crumbs or a sticky spill. It might also be a pantry, food cabinet, or even a small appliance with food residue.
An ant scent trail is invisible to humans. The easiest way to see a scent trail is to watch the ants traveling in a line. Very rarely will a few ants break off from the line to scout new food sources. Early on in an invasion, the unbroken line to the food will mark the pathway of the scent trail.