Do you have a photo/description of the beast or just holes?

Is the damage extensive, moderate, or minimal? A small amount of mysterious bug damage is to be expected in any but the most heavily chemical-ridden garden.

There are a number of things that eat beans so we'll need some more details to give good advice. :-)

I would definitely check and see if slugs may be the culprit. With the wet weather in many areas, slugs have been having a field day! I had a similar problem earlier on in the year with my bush bean crop. If you never see bugs on or around your plants, and holes are in the middle of the leaf, not from the edges, you have slugs. Go outside with a flashlight around 11:00 to midnight and see if there are any slugs eating away. Garden centers have lots of products that could take care of the slug problems. Also, you can spread egg shells around the bean plants, or put a small dish of cheap and fresh beer out at night and check in the morning--slugs will crawl into the beer dish and drown. I hope this helps!