When we hear of greens being eaten, our first thought is slugs and snails. But, from my experience, in a greenhouse, mice are a major problem and can mow young plants to the ground. I have also noticed that red greens are generally less damaged, I have seen that with cabbage, kale, red lettuce etc. I won't say they will not also be "chewed" on, but they are not first on the menu. Patrol for slugs and snails first, checking the pots bottoms, if in pots, inner leaves, lower leaves, undersides of leaves. Look for the characteristic slime trails. By the way, if the area is quite wet, they will not slime as they can already slide along nicely. For control, the commercial iron-based molluscicides do a pretty good job. They do need to be reapplied as needed. Let me know the result of your detective work.