Cabbage must be a treat to all. There are sure a lot of insects, worms, aphids, slugs, snails, etc. that love to eat it.
A good visual inspection may turn up the culprit. Slugs and snails come out at night.
A few holes in the leaves is not going to hurt the plant. You need to watch closely though to see if the problem gets worse. Sometimes these problems can proceed very rapidly and ruin the crop if you don't catch it in time.
Bt, and diatomaceous earth are good natural controls for many pests. Aphids can be discouraged with a mild soap solution. There has been discussions on soap on here before. Do a search. Good luck!
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-