CBP apparently trains dogs to detect both firearms and currency. If a canine can be taught those two things why not other pairs of items?
https://www.cbp.gov/border-security/.../disciplines-2
Canine Currency/Firearms Detection
Canines are taught to detect concealed U.S. currency and firearms.
Both the Officer/Agent and canine are taught the proper search sequences when searching vehicles, aircraft, freight, luggage, mail, passengers and premises.