I read a book on cadaver dogs. Most can pick up scent from long buried bodied and some even from bodies in deep, cold water. Their sense of smell is greater than their hearing.
A dogs sense of smell is truly an amazing thing. They can not only smell increadibly faint smells but can isolate different smells in a mix of things. Masking a bomb smell for example with something like garlic or onions or meat etc doesn't work as the dogs can separate out the different smells.