It might be flea beetles, in which case they'll pockmark the leaves but not do any real, lasting damage. Just keep a close eye on the plants, check them throughout a day, top and bottom, you might see them, they're as small as the period at the end of this sentence.
I get those mystery marks myself on plants. Frustrating when you can't see the enemy.
Caterpillars, and moth larvae will sometimes do similar damage, and can be hard to locate as they are often the same color as the leaves. Check the stems of the leaves,and even down the plant stem, as they can hide during the day. Look for their droppings as well which are usually quite obvious.
The cabbage moth is my worst enemy. Their larvae can really do a lot of damage in a little time.