Regardless of the detection threshold for dogs, I've seen posts generally divided into 4 categories. Those who walk by dogs with big bags of pills and nothing happen, those who actually get busted by dogs at border searches or whatever, those who have absolute faith that dogs can detect single molecules of volatiles, and people who say dogs are no better than guessing. I am more inclined to believe the latter than the former 3 people.
As far as I know, most dogs at festivals or other events are not sniffer dogs, but more of the 'crowd control' type. This was told to me by a woman that claimed she had worked for 12+ years with sniffer dogs (actually, just one sniffer dog since they are coupled to one or two officers to establish a bonding relationship), but of course she could have been talking out of her ass just to look interesting. Alas, she explained to me that crowd control dogs are trained to observe aberrant or 'guilty' behavior. Based on the four explanations you describe, it fits the 'just guessing' explanation best. Sniffer dogs are far too expensive and sensitive to deploy near events, where the biggest possible bust could perhaps be worth $ 10,000 or less.
As far as dogs being able to detect single molecules; even the human nose is capable of detecting certain molecules at the mg per kton (0,001 gram in 1,000,000,000 gram) level, and a dog's sense of smell is about 100 million times stronger. This definitely approaches the single molecule barrier (20th order of magnitude, Avogrado's is 23th order of magnitude), at least when looking at order of magnitude. But not only is a dog's sense of smell much better, their brain is also dominated by the olfactory cortex (where the human brain is dominated by the visual cortex). So a dog perceives the world not visually, but olfactory.
On the other hand, I have given it some second thought and perhaps you are right that a dog does not smell the LSD but rather a chemical that is formed by breakdown of LSD. But they are not specifically TRAINED with the breakdown product, rather with the LSD.
Lastly, about the melting point and vapour pressure you mention... Even kitchen salt (vapour pressure virtually non-existent, melting point above 1,500 degrees? [centigrade, of course]) has a smell to it if you sniff it from a 500 gram container. Just to show that vapour pressure and melting point are only approximations; a lot of substances slowly evaporate far below their melting point. Then I can't really imagine any chemist in the drug-business being able to produce a salt with absolutely no remaining bits of the free base, but I am not an organic chemist so I might be wrong in estimating how hard it is to produce a 100% pure salt.
