I sold bees to guy last summer who called me claiming that they gathered so much pollen from rhubarb in one day that they had no room for it and it was everywhere in box. when he went to install the nuc in a hive body he ended up dumping a bunch out of nuc box onto ground. I dont know if he was nuts or what but I just went along with the story. Maybe some times they dont have room, groom off pollen, then housekeepers take it out like trash. Probably spoiled pollen though or entrance theory.
Just thought Id mention it.
More likely, what he thought was pollen was old cappings.
If the entrance hole is less than 3/8 inch wide/around there is going to be a good amount knocked from the pollen sacs as the bees reenter the hive. Bees will not usually reclaim pollen discarded at the hive, whether done some by accident or design.