As a rule there are 2 types of sniffer dogs - active and passive.
Beagles or Spaniels are active and are used in the baggage rooms etc, where they will bark excitedly if they smell narcotics. They are also commonly used at border patrols.
Labradors and German Shepards are the passive kind, usually used to catch drugs on the person. These passive dogs don't usually bark when they smell narcotics, they are more likely to sit down next to you or glance up at you which signals the customs officers. Sneaky.
Note that nearly all sniffer dogs are trained to identify all out of the ordinary smells. They can differenciate between all smells, whether it be specific plant matter, drugs, fruit or irregularities such as 'masking' agents. Peppermint wouldn't be any different.