DNA doesn't go away in certain areas. I have seen in cases where tiny bits of blood spatter or other biological fluid was found years after the crime. It would still contain the same DNA DNA on the stairwell after having been painted is a bust unless it dripped into a place where paint didn't reach and has been there all these years. Yes. A cadaver dog would find a dead body (or should) even after so many years. The initial odor of a cadaver goes away for humans, but not to a trained dog's nose.