Because lactose is cheap, mildly sweet, and useful for texture and browning, manufacturers add it to many foods that have no obvious connection to dairy. Common examples include sliced breads, crackers, biscuits, flavored potato chips and crisps (where it can act as a carrier for seasoning), instant soups, sauce mixes, processed meats and their vegetarian imitations, breakfast cereals, and margarine-style spreads.
Chocolate is another frequent source: milk chocolate contains it by definition, but many dark chocolates also list lactose, milk fat, or milk powder, or are produced on shared lines with milk chocolate.
Pharmaceuticals deserve particular attention. Lactose is one of the most widely used excipients in tablets and capsules because it compresses well and is chemically stable, and it also appears in some dry-powder inhalers as a carrier. Drug labeling lists it among inactive ingredients. People following a strict vegan practice generally treat necessary medication as an exception, since alternatives are not always available. Anyone who wants a lactose-free formulation can ask a pharmacist whether one exists for a given drug.