So, I think initially they only used SAR dogs, and that is what led them to believe the boys had never spent time outside of the doors of the house. these dogs do NOT "hit" on cadaverine, whole different training.
On the other hand, could the FBI have been using cadaver dogs in their back yard search? I am sure the cadaver dogs went in that van, I just have not seen any indication that they might have had a cadaverine hit anywhere.
"truth is within the lies" is definitely a real thing, seeing between those lines can be tricky though. I wouldn't be surprised if after their interview where they referenced the yard 100 times the FBI had really thought there could have been something of evidentiary value somewhere back there.
too bad the police chief said they found nothing helpful in that search! moo