From my memory, Allison B C case had forensic evidence gathered of “vegetation” foreign to the location where her body was found, but in abundance at her home.
If you consider a “forensic” search of bush land it could be that anything, even any plant growing, any soil, any deposits of oil or other substance in that area, foreign to that area, may be evidence.
So if someone has been in that bush in the last few years, it will be proven they were there. Even if it’s just a few grains of sand from their shoes or a certain part of the beach.
My guess is LE know who they want to prove was there, because they have already said they have never been there.
Real life CSI type stuff here, someone has to be be getting very nervous. Check the resources being put in, they are not having a guess here, evidence collection is to prove something...