How scientists documented the extinction of a sub-species of Indian rhino

How do we know when a species no longer exists in a given area? The process, as explained by Brian Switek, is grimly fascinating. Scientists used dogs to find piles of rhino poop between October of 2009 and April of 2010. They found 22 dung piles, but DNA analysis showed that 17 of those belonged to a single rhino. After that rhino was killed by poachers, the rhino footprints vanished and no new piles of scat appeared.