Could be slug damage. They hide during the day but some people go out with a torch on damp evenings and pick them off. Or you could simply use pellets if you don't object to them.

Surely there would be slime trails evident if it were slug damage? Also many of those leaves, on the peas for instance, seem a little small for slugs to be crawling around on. I'm not convinced it's slugs. I've seen no sign of any creatures really, leading me to believe that there is something minute eating them from the inside out. Can anyone comment on this theory?

There are different sorts of slugs. I get these little holes too as well as big ones - they seem to be made by very tiny black ones and no slime trails to be seen. Slugs often don't leave them. I do really think that's what it is. Nothing to lose by putting some pellets down while you're waiting for another theory.