Maybe it's a caterpillar? I have had beautiful butterflies (black swallowtail) leave their evil spawn in my herb gardens, and destroy them within days. It's amazing how much damage just one of them can do. I now check daily for eggs to catch them before they get me.

I doubt a caterpillar can cut full size lettuse leaves straight off? Perhaps the broccoli and carrots...

Fall planting has not been going well, pest pressure is very high...

Sounds like rabbits or voles, to me.

Again, I managed to get some big beautiful fall lettuce, gone, broccoli gone, brussels sprouts, and carrot greens, all eaten :(

They don't seem to have eaten anything else. Not interested in the peas, all I have left for fall now, or the tomatoes, or squash, or chard.

Mice? I don't see any signs of digging, can a groundhog climb a 6 foot fence?

How big are the openings of the fence fabric? Is the lowest part something like 1/4 inch mesh hardware cloth or similar or is the fabric more like 2 x 4 inch welded wire? Even fence fabric with a 1 inch mesh is not small enough to keep a vole (mouse) out.

It is I think 1"x2" mesh. There is no way it would keep a vole/mouse out. Maybe I should add a skirt next year.

A skirt of 1/4 inch mesh hardware cloth might be enough of a deterrent, although I am not sure how high a vole, or other critter, might climb.

You know I take it back... would a mouse eat an entire salads worth of lettuce? I don't see any signs of diggin g though. I will see more when all the weeds are gone. Def think I should add a skirt next year.