I have been puzzling over this for several days. At first I thought that the lesions resembled feeding damage where whatever insect pest has been scraping away at the top surface of the leaf. But you say that there doesn't appear to be any insects attacking the plant. My next thought was that these could be white rust pustules. The fact that it is on kale and spinach, both related species, is likely a clue. Are there any symptoms on the leaf undersides? Do you think you could upload some extra images? It's very easy to click back into your original question and attach more. A close up of those lesions and also some of the undersides of the leaves would be helpful.