I'm sorry to say it, but somebody has to, the reason this was not found by the LE "experts" is they were not thorough in what they were doing. It looks to be as simple as that.
In this interview:
http://forensicsource.blogspot.com/2011/08/exclusive-interview-with-sandra-osborne.html
Sandra Osborne describes what she did and did not do in examining the hard drive on the particular computer in question. She did NOT know what she was doing in trying to examine the Firefox data - she talks about it in the interview. She had to have her boss retrieve the Firefox data.
As AZlawyer wrote in a previous post on this thread, she was "close, so close". But after reading this article,IMHO she had neither sufficient education or experience, at least at that time, to conduct a thorough forensic evaluation of that hard drive.
The big question I would have is why was her work on a case of this magnitude not peer reviewed? By an outside expert, of course.
If any good comes out of all this, it should be that LE Agencies need to hire and pay qualified IT educated professionals to do this type of work. Sending someone out to a 72 hour class to earn a "certificate" does not a "forensic computer expert" make!
Sorry if this post sounds snippy, but honestly, all I can say is read the interview and you will see how this happened. Obviously Baez's computer people didn't have a problem with the data and neither did JWG.
Here is an article posted by the computer expert who found the data for the defense:
"Probably the most disturbing thing I saw was Sandra Cawn-Osborne's response; "I wasn't told to search for suffocation." One thing I will say to that is that she was not alone in the analysis of the computer forensics, she had help from her supervisor, who was a teacher in the Digital Forensics Master's program at the University of Central Florida. Ponder that one."
http://exforensis.blogspot.com/2012/11/casey-anthony-google-searches.html
He doesn't think much of Nancy Grace.