I think they got rid of a big section of worker sized foundation purposely to make room so that they could build some nice drone comb on the side of the box right there in that spot. When bees feel a need for some drones they will have some! If you want to avoid this in the future consider giving them one frame of foundationless or of drone size foundation, and let them raise a few drones. Then they won't be desperately creating burr comb in weird places just to raise a few drones. I have a dedicated drone frame in every 20-frame hive and I have had no burr comb at all in 3 years in any of them.The odd thing is that they carved these holes in the foundation and then built burr comb on the hive body wall adjacent to the holes.