If you are not finding catepillars then my guess is earwigs. To make sure that is what it is dampen two sheets of newspaper and roll it up to form a tube. Put thbe on the soil near the plant in the evening and then unroll it in the morning. If it's earwigs you'll find them in the tube. You can just roll up the tube and put it in the trash. Doing this nightly should help you keep them under control. If you want better control check the pesticide labels for use on green beans and for earwigs.