ITA!
I don't think it was a coincidence that it happened like it did. They knew something when he threatened his girlfriend's sister with a gun and a knife, imo.
Now wouldn't it be something if during that case they obtained a SW and confiscated the gun and knife he used in the threat and they still had evidence on them of Holly's?
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.