A few days ago I saw a Bumblebee nosing around the entrance to Hive D. It was going in and coming back out again without being visibly challenged by the guard bees. I was surprised that a bee which is so obviously different to my eyes could be ignored by the guards.
We're told that the bees use smell to establish which bees are allowed in. This colony specific smell comes from the honey in the hive and the nectar coming in. They all pass nectar/honey around between each other so they should all smell the same. This, I suspect, is why robbing goes unchallenged once it gets established - the robbers smell the same as the robbed. (as mentioned in...)
I suppose that stored honey varies in smell depending on which nectars were available when it was being made. I wonder whether there is confusion in the hive when all the bees rush to fill up on honey during an alarm (like a hive inspection with smoke)?