Bomb-sniffing dogs have their limits:
Dogs get tired, and less reliable, as the day goes on, just as we might. As for conditions, odors change in the weather, and dogs do, too. When they’re panting, dogs don’t sniff as readily as they normally might; an overly warm dog, forced to pant to cool himself, has a less reliable nose. And while we don’t think of odors as seasonal (until, upon reflection, we remember: there is a smell of spring, of summer; the odors of winter are fewer), smells are more volatile in warm weather, and travel and disperse differently than in the cold. This too affects detection rates.
Combine all of these factors, and a scenario like the marathon bombing presents ample challenges for a detection team: a densely peopled environment full of movement and scents, dogs that have been working for hours, and explosives that weren’t stationary until almost the last possible moment.
The Dish 