I cant figure it out either, its been in my head and frustrated me for months. If the dogs hit on decomp in the backyard, WHY hasnt LE dug it up or searched it again.
My understanding is the dogs can hit on very little decomp so it could be KA had left something back there besides the body that had decomp on it. Say she sat her shoes down for a little while and then removed them later-thus leaving a little decomp. A tarp or blanket would also transfer decomp.
Also LE would be able to tell if the gound was completely dug up. And I am almost sure the would check under the sandbox. No I would be more worried about main structures or fresh concrete whether they completely searched those areas
JMO of course