It's most likely a night time visitor. Something that lives underground and comes up at night to feed - A worm or some kind of grub.

I would say its flea beetle, they love members of the brassica family. We used to use Gammexane Compound named Lindane, quite rightly banned long ago.

Agree with Jimmy, flea beetle, I'm not sure how you treat now.

Looks like a clear areas in the leaf and not definite holes. If that is the case you have leaf miner problem. Since this is an edible plant I will make no suggestions on a chemical control.

Scattering citrus peels around my veggies seems to work for a while anyway - until it decomposes.

Planting a little lemongrass is another natural way to repel the bugs without using chemicals.