Right then. Research is a wonderful thing. (I actually used to make a career out of it!)

Guide to Honey & Bees - Ted Hooper. (the "bible" according to many)(actually, it is pretty good!). Chapter 2, The Bee's Behaviour - section = Guard Bees.

"The colony odour is a product of the food eaten, and because of the very thorough food transfer all the bees in a colony have the same substances in the same proportions in their gut, and so their scent is the same. Different colonies will have different odours because they will have a different mixture of the various flowers of the district." (Hence the guards distinguish between colony members or robbers/drifters).

So you're smelling the selected flora from about 2 miles radius. :-)

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