I see comb taken back to the mid-rib when either the comb has sustained wax-moth damage, or in order to remove dried-out and hardened pollen. In your photo, neither of these appears to be the case. So - it's guessing time. 
I notice that on the bottom bar the bees have made a vain attempt at drawing drone cells - perhaps that was also their intention by removing sections of the existing comb ? Only guessing mind ...
Do you have drone combs in that hive ?
LJ
Just re-read your post - perhaps there were swarm cells in those areas ...
I notice that on the bottom bar the bees have made a vain attempt at drawing drone cells - perhaps that was also their intention by removing sections of the existing comb ? Only guessing mind ...
Do you have drone combs in that hive ?
LJ
Just re-read your post - perhaps there were swarm cells in those areas ...