Why sniffer dogs only detect `real` explosives

Have you ever noticed that sniffer dogs at Metro stations or airports do not bark at you though you may be carrying a few things that smell similar to explosives - like a plastic grocery bag?

Customs department facing shortage of sniffer dogs

The Customs department is facing a shortage of sniffer dogs to detect drugs and counterfeit currency.

Delhi Metro to deploy new squad of sniffer dogs

The Delhi Metro will deploy a new
squad of sniffer dogs in order to increase vigil and thwart
attempts of explosions on the rail network.