There is a forensic paper where they tested if K9 dogs can detect pharmaceutically pure methamphetamine and the result was: No. They can not.
Reason is simple: The dogs are trained with real-world seized drug samples not pure drugs. So the signature of the drug is usually much more determined by solvent residues and volantile impurities then the drug itself.
Also dogs cannot find in a normal environment very well packed drugs, how could they? A professionally sealed package of cocaine which was cleaned outsides, re-sealed in a absolute drig free environment and packed into 100% not drug contaminated luggage handled by people not being in contact with any drugs will have a far lower signal strength then any 50 € note used for sniffing cocaine at any time. And those contaminations are everywhere in normal environments and so the signal to noise ratio is just not good enough.
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Frit Buchner is right though. Most drug sniffing dogs you see are not even trained, it works like this: The dog sniffs your luggage, you watch the dog and somebody a bit to the side you did not even notice is watching you. The search comes depending on your reactions to the dogs sniffing and not from the dog. If you tension up when the dog sniffs a certain part of the luggage the handler gets a signal and signals the dog which sits down (usual they are doing this when successful, if it barks it was no trained dog for sure). Then they pretend the dog told and they search and 90% or more they wil have exactly the spot. Which you gave away.
When they send a dog to sniff your luggage: Do NOT watch, turn away. Why should you watch you are innocent so no need to look. Thats the only way.
Nowadays the electronic swipe devices able to detect traces are more dangerous anyways and they are used the same as dogs are. Be assured if customs wants to find "traces" they will find traces. As "traces" are by definition a "not quantifiable amount" of something. Very close to the realm of Homeopathy actually.