I was recently in an airport for the holidays and happened to be in a line that had a drug-sniffing dog run past us.
I am around (and usually covered in) cannabis all day most days, whether tending to or trimming it, and when I was taking this flight I hadn't had time to wash clothes beforehand, so all of the clothes in my bag were utterly contaminated with cannabis plant residue (so much so that when I got off the plane and my buddy picked me up, he could smell it and thought I had brought bud with me on the flight!).
The dog ran right past everyone else without a second sniff, but ducked his snout in right in front of my bag, but then went right back to his trot running down the line of passengers without alerting its human counterpart. I don't recall what brand of dog it was, but it came in a small form factor!
This begs the question... how sensitive are drug sniffing dogs to the actual substances they're sniffing for? I can only assume the dog was smart enough to actually know there wasn't enough quantity of dope to bother alerting. Are dogs trained this way, or was this dog an anomaly, and is this information useful at all in determining how to more efficiently ship packages? - ie. it's helpful if I don't need to bother changing clothes and taking a shower before I can properly prepare a package for mailing...
It was most likley a bomb dog trained to sniff bomb making chemicals and residue. They don't cross train so it will be on one duty or another. To answer your other question their sense of smell is amazing.