After a day or so of rain, some of it heavy, the bladder area filled with water! The water was about 8 rows of bricks deep, which is much more than the amount of rain. I presume the water came from the concrete slab, and perhaps also some ground water, although I suspect that was a minor component, given the water level was above ground level!
Note the slots in the wall to locate the steel supports for the Bondek. The supports were, I believe, ordered for doorways in the house, but were not required, and thus were "modified" for this role (by having an end cut off).