Assume Gannon left with tee and did not come back and assume she killed him during that 4 hour window.
That would mean she did it in broad daylight. So, she'd HAVE to have cover of some kind.
So, she either drove to a remote spot known to her and did it.
Or she killed him in the truck, and hid him somewhere.
Or she had the cover of trees in a country setting where she'd not be seen.
I can't see her putting his body in a creek along the road, or a culvert, or a lake, or even a dumpster. Remember, it's daytime, like, the MIDDLE of the day. Too much risk along a traveled road.
I think she drove off a main road, up a farm field path, in to a wooded area, or up a dirt road that leads in to the mountains.
She drove somewhere FAMILIAR to HER. Somewhere she'd been before. Somewhere she KNEW would be secluded enough to hide his body.
Now assume she moved his body.
She STILL had to access a remote spot, that provided enough cover to kill him without being seen, then return to move his body elsewhere, somewhere more remote. This theory could provide her with cover of darkness, but she'd also be working in the dark, where she had previously left him, likely needing lighting of some kind, headlights on, or flashlight.
This all sounds too risky, too cumbersome to me. Unless she returned to get his body and move it during daylight hours. But, it still involves quite a task, and one that would be risky.
LE appears confident Gannon is dead. So, they have a body part, or some other matter, or definitive proof of some kind, that he is dead. That proof came from where?
One of the search sites? IF YES, then I believe she would have had to leave evidence of his murder at that scene prior to moving him elsewhere. I just don't think Gannon's body is at or near a site that's been searched, as I feel it would have been found.
In the truck? Possibly, killed him in the truck. Dumped him in a remote location. All in that 4 hour window. Proof of his death in the truck, no body yet found, search sites all providing evidence discarded after the murder.