Can you figure out where they came in and plug the opening? they can get through a small hole or even through a patio door. They can be attracted to lights on at night.

I rented a house years ago that had a billbug infestation. I remember killing many each day, and this went on for at least two weeks before I realized there was a problem. We pinpointed the location where they seemed to be entering, and removed the baseboard. This was a house on a slab. Behind the baseboard were thousands of them. Gross! We cleaned it up and sprayed. Also, we removed plant growth on the outside. The problem was resolved then, but I don't know for how long since we moved soon after. :)