When we used to live in Spotsylvania,one our friends worked for the Sheriffs Dept,he trained this little dog in classes.It was just a little mutt dog and was so good at tracking,it became one of the best little tracking dogs they had.IT wasn't a cadaver dog,just used in search and rescue.I think he got it in a shelter or someone gave it to him,can't remember.
I watched a program on how a dog was adopted from a shelter and taught to be a search dog. They described the dogs that love toys are the best one's to train.