I'm not convinced that the threats did anything at all for information about Holly, other than open the door to his property. When you have suspicions, once you're in the door "looking for what you are there to search for," you can follow up on ANYTHING you "accidentally" come across in the execution of that search. And the evidence from one search can lead to permission for another. Given his profession, this creep's pseudo-mentor was probably Walter White on Breaking Bad, and that show may have also been his textbook in how to dispose of bodies where they won't be found. In one search or another, LE could have found "tools" used in the implementation of such methods.