Some kind of chewing insect. Possibly a slug or snail or caterpiller. Look for insect eggs on the bottom of the leaf, any slime trails, insect frass (manure) etc. 
 https://ag.umass.edu/sites/ag.umass.edu/files/fact-sheets/pdf/whats_eating_my_vegetables.pdf

So what should I do about this I can’t see any of the things you mentioned but there are other leaves apparently being eaten as well.

The responsible insect should be identified before any action is taken. If it is a leaf cutter bee, then the damage should be tolerated because the insect is a pollinator. If it is some other kind, such as a Japanese Beetle, then the beetles will be leaving soon. How much damage is being done, on one plant only or on several? If the plant has so few leaves left that it is in danger of dying, then something may be done. If it only occurring on a few leaves, then tolerate it because it is near the end of the growing season. What kinds of plants are being affected? Because the damage from several insects may appear similiar, it may not be possible to determine which one is the culprit.