Possibly the flea beetle - you won't find it because, as it's name suggests, it hops away quickly when disturbed. Although unsightly I find the damage isn't usually enough to need any action.
Companion planting apparently helps - it is suggested that you can grow thyme, catnip or mint to cover up the scent of the veg plants - I've not tried this so can't say whether or not it works.
It is also suggested that sowing a row of radish nearby will lure the beetles from your veg to the radish leaves - as you don't eat the radish leaves the flea beetles can be left to chomp on them happily, and of course the radish roots will be fine to harvest.
There are lots of different types of flea beetle - some are beneficial as they target some weed species. And they also have their predators in the insect kingdom, so if you don't use pesticides in your garden there will hopefully be some of the right insects to scoff your flea beetles.