> ...i thought the term "pheromone" specifically 
> referred to a sent produced by an animal.

Entomology does not always respect etymology,
but bees make wax and also make the pheromone
in the wax, so the concept can duck inside
your strict definition.

> is the smell of empty comb really distinct from 
> that of comb with something in it?

Yes. I think Gould and Gould covered the whole 
series of experiments with comb to verify the
influence of comb on the colony.  Seeley also
mentioned some of these experiments in
"Wisdom Of The Hive".

The specific impact of foraging enthusiasm is
pretty clear, so the bees can detect the difference.

Pretty darn good for a brain the size of a
pinhead, huh?

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