Slugs, snails... could be pigeons - can they get in?
Have you lifted the leaves at night and looked under them with a torch? Some bugs/caterpillars are very small and often the same colour as the leaf.
Cabbage Maggots
Cabbage maggots begin their cycle in the early spring, when small grey flies emerge from an inch beneath the soil and begin laying eggs. In less than a week, quarter-inch yellowish maggots hatch and begin feeding.
Cabbage Flea Beetle
Flea beetles are about the size of a pinhead (but these leave tiny holes in the leaves so unlikely it is this).
Only other thing that springs to mind are leaf cutter insects.
I notice you have a uniform little 'nick' in several of the possibly 'cut' areas, have you tried google to see if you can match the photo?