Superhuman sense of smell is an ability possesed by vampires. By using the power, vampires can smell almost anything.
In Fanged and Furious, Sarah was able to smell that Malcom had cut himself when he had fallen. Later in the episode Sarah, Rory, and Erica were able to smell the blood in the hood of the car and were also able to tell that it was human.
In Flushed, Sarah and Rory were able to tell that the alligator had been up the stairs.