The pmi for remains of a body enclosed in a plastic bag could best be estimated by

The post mortem interval of a body enclosed in a plastic bag could be best estimated by the insects found in the body, in that case forensic science need to listen to what those little creatures have to say, insects could tell if the body has been moved from the crime scene, how long it´s been in an specific place, and how long it´s been since death until found, every group of insects can tell something different, from worms, arachnids to flyes, each one of them has a time to get into the body, so it could tell you for how long it´s been dead.

Gravitational force i believe