There's this article that I just read this morning. It suggests there are some valid ways to distinguish growth changes in plants caused by dead bodies and those caused by dead animals. Regarding your point, decaying dead bodies in the case of murder will typically be buried, whereas dead animals will die above ground and be scavenged, resulting in far fewer nutrients being absorbed by nearby plants. Of course that would limit its application to murder in comparison to missing people in general.

https://www.economist.com/science-and-technology/2020/09/03/...