The smell thing will be very annoying. While parts of the area will distinctly smell like Cassian, the exact place where the murder takes place will smell like... whatever you imagine an interdimensional abomination against all life and sanity would smell like.

Not like pumpkin pie.

Cassian himself smells like old magic. Bone dust, book glue, leather and iron. That will almost always be found within a mile of the murder site, but the closer it gets to the actual place of death, the more the human scent will be replaced by Hastur.