Well, take a hound dog, for example. He has many, many, many more olfactory cells in his snout than humans. Imagine you and a dog are hiking along some unmarked trail. The dog, unbeknownst to you, notices the smell of gas. Maybe it bothers her, and she decides to walk away from there. You follow the dog, away from the trail, and discover that a couple of hours later, there's an explosion at the exact spot. There was a leaking gas pipe a dozen feet below, and the dog had noticed it.