A caterpillar of some kind is the most likely suspect. Some (like Tomato Army worms) start off very small and are hard to see at first. I spray Bacillus thuringiensis and it works really well. It is a caterpillar specific strain of bacteria that is basically inert to people, animals, and beneficial insects like bees. They have to eat it to be affected, so you have to get total coverage of the top and bottom of the leaves.
