Very common, and I have no idea. Every spring (and to some extent later in the season) you'll see bees doing odd foraging on rotten leaves, compost piles, etc. I don't think anyone has gotten the bees to give up their reasoning, but they do orient on odors so strong-smelling things like grounds might attract along those lines. They don't seem to actually forage the grounds in my experience (like packing them in their baskets), just sort of scouting/licking them. Other ideas could be mineral content or maybe they think it'll be a propolis source?
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