I picked up a swarm yesterday afternoon & brought it home. This morning I removed all of the foundationless frames that were used to support the frame of brood that I used. I replaced them with extra frames containing some more brood as well as some frames with fresh foundation.
Upon removing the empty frames, I noticed the solid floor was covered in opaque wax flakes. I assume those flakes will get picked up, mixed with bee saliva, then fashioned into honey comb (as in the video: City of Bees), which had already started. I reckon that all of those flakes would have fallen through a SBB.