A scout bee is sent out to look for abundant food, and may come upon the weakened hive. They enter grab a mouthful of honey and head back home. Then they send out directions to the other roving bandits and before you know it, your hive is completely surrounded. It will be a battle royale. It was with ours, it totally decimated our hive.
The hive entrance looks nothing like a normal function. There are bees flying chaotically all over waiting for their chance to gain access to the hive. They are fighting, stinging and biting each other. There will be dead bees all over the ground at the front of the hive, and if the robbers gain entrance there will be dead bees in the bottom of your hive. They are loud, and the tone is not the normal humming sound, it's definately turned up a notch and you can tell there is anxiety.
Unfortunately, once they know they have access, they will continue to come back until everything is gone. We will be moving our hive to a new location when our new colony arrives. We don't need to make it easier for the same group of bandits to find our hive again.
Even a well established hive should have an entrance reducer installed in the fall to help avoid robbers.
I've also learned that this is a totally natural occurrence. If you see the robbery in progress you can try a couple of things; put a wet sheet over the hive. Your colony of bees will manage just fine, and allow them to handle only a few robbers at a time, it also makes it more difficult for the robbers to get access to the hive opening. You can also spray water on the bees, not hard, but like rain fall. Some of your bees will be in that chaotic scene. You're basically tiring them out. I've read a couple different articles both positive and negative with this option. But if you're desperate, you may want to give it a try.
I am disappointed with myself for not knowing this information. Hopefully it will help others. I know that beginning beekeepers have a hard time getting a colony established. But we'll keep our chins up and try again just as soon as we can get another hive. Live and learn.