these are my thoughts...........robbing.... your entire hive will be engulfed with bees trying to get in, and getting in, and many bees fighting, and lots of dead bees on the landing board plus wax bits. if it is a weaker hive, you will see the initial frenzy, and many bees coming and going, and again the wax bits left on the bottom board and signs of ragged chewing of capped frames on the inside. sometimes when we think there is natural forage available, there is not. the only way to determine is looking on the inside and not what is blooming on the outside.
with that said, i think what you are describing is bees will migrate to other hives, whether it be from your queenless hive, another hive, or other hives from other keeps that maybe nearby. if the incoming bee does not have pollen or nectar when she is coming into the hive, she will get more than a cursory examination. (bees coming in with nectar and pollen are 'allowed' into the hive). without nectar/pollen or 'foreign' bee, the bees will roll and fight, and she will not be allowed to enter, and she is carried off. i think this is what you might be seeing?
if you are unsure, and you think robbing is occurring it does not hurt to put entrance reducers on.
helps if you can take a photo of what is going on,or better yet, if you go through the hives you will see ragged chewed cappings on the honey frames and bits of wax on the landing board.
