We know AA was in the canyon at that location on the 25th, as indicated by his phone pings.
I find it almost impossible to imagine how he could undetectably store a charred, decomposing body over the previous @1 week. Especially in June – it wasn't as though he could preserve it in a snowdrift. The idea of him leaving all of her skeletal and tissue remains in the burn pile for days, in full line of sight (and smell) of neighbors' houses just a few feet away over the fence, then digging almost everything up and transferring it to his car, beggars belief. Of course, everything to do with the burn beggars belief, but if a neighbor complained of the burn smell surely the after-burn smell also would be noxious?
Perhaps the "white cabinet" visible in the footage of the garage search was a freezer? Or he had one in the house, that we haven't seen?
Or perhaps he visited the canyon days earlier to carry out the burial - late on the 18th, or maybe the 19th – and his trip on the 25th was a follow-up to check on the condition of the burial site, or simply to revisit the thrill of his kill.
Maybe he didn't take his phone along on an earlier trip. (Hey, dumb luck could strike even AA.). Or maybe he did, and LE hasn't revealed that bit of info to the public.
He was only there for 2 hours on the 25th. Not much time to dig even a shallow grave in hard rocky soil, then fill it in and (possibly) attempt to conceal it.