How Did a Dog Miss an Explosive Ordnance Disposal?

How is it that a dog trained to sniff black powder and explosives from yards and yards, yards away, missed it when it was sitting right there in front of the d n c? How is that possible? Then, you know, we're here, so you can answer it first. We have to folks, wit oum's razor. Keep it simple, stupid accept the most plausible explanation that doesn't require you to assum a lot.