Cabbage white caterpillars can do a lot of damage in very little time. Check the tops, crooks, and undersides of leaves and be sure to be really thorough - cabbage whites can vary in size from a few millimeters to an inch or two, and they're fantastically camouflaged against the greens of the brassica plants they love so much.
When you see, later in the season, little white butterflies in the garden, watch where they land. They lay their eggs on the undersides of cabbage, kale, cauliflower, broccoli and other brassica leaves. These eggs are very tiny ovals, yellowish in color, and they stand on end. It's best to get rid of infested plants by throwing them in the garbage, not your compost; most home compost piles don't generate enough heat to kill pest eggs or larva.