Sitting there watching TV we could hear scratching every so often, but never really put it together until one evening I was outside and saw hundreds of bats come swarming out of a 4" crack. Hundreds. of. bats. They'd fly around for a bit and then go back in there. One evening when they mostly had left, we rigged up a metal flashing to close up the gap and attached it to the 2 walls, and installed some industrial grade glue to the flashing to fill in the gaps.

Apparently they weren't all out of there, because soon after we applied the flashing and goop another 50 or so came out of the opening, but now they had glue on their wings and couldn't fly very well. They were dropping onto the driveway shrieking in pain and frustration. I must have dispatched 50 of them with a shovel. Then the others returned and tried to get back into the wall, but couldn't due to the flashing. They circled me as I smashed their family members on the concrete.

A very strange night. The next day I affixed the flashing permanently and it was only a few days until the shrieking and scratching inside the wall by those still trapped stopped.....