Well I am far from being an expert....
Ex; One that was and is no more
Spirt; A drip under pressure.
Anyway, You are probably right, as I mentioned I couldn't stay focused on any individual all the time she was buzzing around, so I don't know if the secretion was done elsewhere. Just the two or three times I did see "the drop" it was about the same spot.
But they could have been doing it in the nearby weeds as that is where I would lose sight for a bit. Or even off to either side as I was looking to see if I saw what I thought I was seeing. I also do not know if it was the same bee, but I figure they have a limited payload.
I would not know how to research this or even duplicate it again as it could have been a fluke with many variables.
OH, one thing you did say "I also can't imagine another natural stimuli that would get 'learned' reaction from them." How about an unknown presents of danger?