What a coincidence. I had the same issue over the past couple of days and was shaking my head wondering what in the world was going on. I had 2 hives (of 5 in this bee yard) with large clusters of bees clinging underneath the bottom screen. They had been there for a couple of days. I have robbing screens on both hives. Yesterday I brushed them off, put in the tray to block the screen, and within a couple of hours nearly all had found their way back into the hives, via the small top entrance on the robbing screens (from Brushy Mountain). So, while I still have no idea what they were doing clustered underneath the hive, I don't think locating the robbing screen entrance was a factor.