For years, the idea of bomb-sniffing dogs has been pretty straightforward: A trained canine walks up and down rows of people, sniffing every person in a crowd to suss out any bomb-making material or explosive devices. But the floppy-eared Labradors posted at recent Metallica, Pearl Jam, Taylor Swift and Bon Jovi concerts have been trained to do something different. They don’t sniff people; they sniff the air.