This happens to me frequently, that one or two hives out of several right
next to each other will experience pesticide die-off and the others are not
affected. I opened two pollen traps this week right next to each other and
the color mixtures were radically different. One had lots of purple pollen
which I assume comes from a large stand of thistle which I've seen bees
working, and it was really pretty. It just shows you how different hives,
especially when there is a large choice of nectar, can visit different
areas. Perhaps someone didn't like your bees on her escallonias or his
poppies and sprayed.