Hoppe's #9: Dog Attractant

In a routine police investigation of the truck, a bomb-sniffing dog alerted while next to the truck, Collins reported, so Independence Avenue was closed from Fourth Street to Seventh Street while the FBI investigated. Part of the Johnson Building was evacuated.

Ok, so a U-Haul truck is parked next to the Department of Education, and the cops do an investigation? Alright, Force Protection being what it is, the truck may have been parked there for some time, and someone thought it should be checked out. I can see that. But what exactly did the dog "alert" to? Some drycleaned clothes? Did it smell gun oil, solvents, powder residue on the "small handgun?" Can police dogs pick up the smell of Hoppe's #9 from the outside of a car? Interesting, none the less.

***ETA: Defensive Carry forums has a thread on this very thing, and the best I can gather out of it is "maybe." Some commenters who handle dogs say yes, and some say no. The glaring question if the dogs can sniff out guns is why don't they alert to every cop with a gun? It's not like the dog hangs out with every cop it may encounter, and is trained to not flip out on the flaky Fed, or the new Deputy. I think it's voodoo.