> It was a warm winter day and a friend and I watched honey bees
> working on a pile of used coffee grounds....
> ...not really gathering it

I'll conclude from the lack of any "pollen packing" onto legs that the bees
did not think they were gathering pollen.  That leaves nectar and water.  

Assuming that the bees were gathering moisture, the caffeine content may
have made them more enthusiastic dancers and boosted the number of recruited
foragers accordingly.  See the article below for the mechanics.

It does not matter if the bees viewed the moisture being gathered as "water"
or "nectar", either one would prompt a dance. 

https://www.newscientist.com/article/dn28349-plants-spike-nectar-with-caffei
ne-and-give-bees-a-buzz/
https://tinyurl.com/ojfd7m5

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