Here's how it works: Adults lay eggs on the beans in the field. The larvae hatch and burrow into the bean, consuming from the inside out. When they turn into adults, they tunnel their way out making little holes. This often occurs during winter while the beans are in storage. In the field the pests tend to only have one life cycle, but indoors with temperature control, they can have a continuous life cycle.
If you are a seed saver, here is a tip: once harvested in the fall and sufficiently dried down beans should be put in the freezer for five days. This will kill any larvae present and hopefully preserve the integrity of the seed.
So how the heck did this happen to me?
I received some bean seeds from someone who has been saving them for years, but apparently they didn't freeze them. I put the seeds with the rest of my collection, thus importing the weevil larvae. Once the adults emerged from the original seeds, they chewed their way out of the bag and moved on to the next bag of seeds, working their way through almost every heirloom bean in my collection.
Here is what my bean seeds look like:
See all the little holes?