Cadaver dogs can smell the minutist amount of scent of human decomp,far beyond what a human could pick up.A huge decompositional event,such as the one in the trunk,is easy for a human to detect.The lighter scents,such as what was on the ground in the A's back yard,only a cadaver dog could detect.The air samples and having the cadaver dogs sniff the car were confirmation that it was ,indeed,human decomp. A dead child was found,not a dead pig,so connect the dots.
If you have ever smelled cat urine you know the difference between it and dog urine or human urine.You just know .Same thing with human decomp,versus any other animal.