I read somewhere about the exact same thing being done with wasps. But with the wasps they make a sound that can be electronically detected instead of the PER. Also something this article didn't mention is that the wasps (bees too most likely) can be trained in just a few minutes by being fed sugar water while being exposed to the smell of the chemical you want them to detect. It's so simple that you could make one and train the bees in your garage.