The bees clean the cells out....feces and all. In a brood producing hive that is simultaneously collapsing from varroa, housecleaning will suffer. When a bee colony goes into a broodless period and is not in iimmediate collapse....those recently used cells will get a clean up.
This is very good to know. I will ad that to my observations.
Zone 5a, Practicing non-intervention beekeeping