Either slugs/snails or caterpillars would be my first guesses. Since you haven't seen caterpillars during the day I'd give the night time probing with a flashlight for possible slugs another shot. If growing in containers it may be worth checking the bottoms for hidden sleeping critters.

If you spray Bt, it's not caterpillars. You might look for slime trails in the dirt that slugs might leave behind. You might consider putting collars around the stems.

slugs or caterpillars, Bt is harmless maybe try again and make sure it is not expired (that got me once).

Thanks everyone. I don't usually have slug trouble since I stopped mulching, but perhaps??? I will try some diatomaceous earth, and maybe sluggo....

That's a good point about expired Bt. Shelf life for liquid concentrate under good conditions is just a year or two. Maybe a bit more for powder. More to the point, once the concentrate is diluted, it must be used within 24 hours. Your damage does look a lot like cats. You need to spray both sides of leaves for best effectiveness.

I find that my monster snails leave chard alone unless there's absolutely no alternative. So the fact that only your chard is affected makes me think it's not slugs or snails.

Btw chard doesn't need to be keep above freezing.

Also a good point, My chard is happy down to 20F at least. I am somewhat surprised that caterpillars or slugs are active in cold weather. Mine go away entirely. I don't apply Bt in the winter around here.

Well, our greenhouse can get down below freezing on occasion, but I find that it grows better if I can keep it just above. I had an infestation of cutworms on my lettuce and mustards, but was able to solve that by picking them off every evening until they are gone. Now it is just this swiss chard puzzle. I have never seen any creature on the plants, even with a flashlight at night - when I was out picking off cutworms. And now ONLY the swiss chard. Well, life is full of mystery. Thanks for all of your ideas, and I will let you know if I figure it out.

rather perplexing since of all the greens, the chard seems to be immune to most pests here in So CA. It is able to hold up to the heat and cold with minimal stress.

I would agree with that. Down here, my chard is adjacent to my collards. My collards get totally devoured by cabbage moths/worms if not for my Bt which is my only defense. Those pests pay almost no attention to my chard.

Do birds ever get into your greenhouse?

Birds do not get into my greenhouse this time of year, as I keep it closed for the heat...