Does the presence of a smell of feces—without the presence of the physical substance of impurity—necessitate washing the anus when intending to pray?

As long as you find nothing on the tissue after wiping, the impurity is not present. Any smells you perceive are merely delusions and whisperings [waswas] that have no reality. Therefore, do not pay attention to them, and do not wash the area unless you are absolutely certain that impurity has exited. We advise you to ignore these whisperings [waswas] and completely turn away from them.