Looks like snail damage.

You don't see them because the sneaky things come out at night. I've given up on dahlias for that reason.

I think the snail theory is right! I have been finding lots of very small flat snails, less than 1/2 inch big all over the garden. How do I get rid of them????

Slug pellets gets rid of them. They find them tastier than the Dahlia but it poisons them instead. You just sprinkle them at the base of the plant.

This year I have only been watering first thing and never after 6pm (apart from last year when I never had to water, I always used to water at night). I have had a lot less slug damage this year. Last year the slugs really went for my Dahlia, even during the day!