Mysterious Holes

Something is eating holes in my beet leaves. I'm not upset about it, I just want to know what's doing it. One of the most satisfying things about my garden is trying to grasp the whole ecosystem: the soil, the worms, the fungi, the bacteria, the insects, the plants...and how they all work together. So I'm looking at this as a puzzle to solve--and not bringing out the saucers of beer (yet).
I've been examining the holes for a few days, and pretty much decided that they are not from any caterpillar-type creature, since I can't find anyone remotely resembling a caterpillar on the leaves, and caterpillars usually lounge couch-potato-like on the leaf they are eating, unable (or unwilling?) to move from their feast. Although I did see some tiny black dots on one leaf, which could be eggs or frass. Could they all have already pupated and flown away?
No signs of leaf-miners, either. I've had earwigs in the back of my mind, and I have cursed them in the past for chewing holes in my plants, but to be honest I have not really seen many of them and don't know if they can really inflict these kinds of holes anyway. (Also I recently learned that earwigs eat aphids so I am practicing kindness toward earwigs.) Sowbugs? Maybe. There are a lot of them around, but again, do they chew these kind of holes? I don't think so. Slugs or snails? Haven't seen any of those or their slime trails, either. There are a lot of ants around, but they seem more interested in the fishmeal I put around the plants than anything else.
This isn't a great photo, but I did find a clump of ants clustered around something on a leaf. But that was the only clump. There were a few isolated aphids on the lettuces but nothing major and none on the beets. Tonight I will go out with my flashlight; if there are earwigs, they'll be out. (Note: I did go out with a flashlight, but all I saw was one earwig lurking on the soil, and no one at all on the leaves. Foiled again.)