If it was obvious, they certainly would have.
My feeling is that they were given specific locations to search by someone who is talking.
Locations that either weren’t looked at closely during the initial search, or ones that required some work in order to examine.
Something like plumbing, under flooring, or an elevated location.
I don’t think it was a coincidence that immediately following this final search, law enforcement said that this was now a murder investigation, and Kelsey was killed in her home.