I think it's likely that when Casey borrowed the shovel from the neighbor, she laid Caylee on the ground near where she was trying to dig. When she couldn't dig a hole in that location, she moved to another area of the yard, moving Caylee to that location. And then, a third location. That's why the cadaver dogs hit on three locations in the Anthony yard. Those are locations where Caylee was place while Casey (and someone else?) tried to dig a hole. Finding that the ground was to hard, or there were too many obstacles - tree roots or bamboo shoots - Casey gave up, returned the shovel, and put Caylee in the trunk of the car.
At that point, Casey would have to seek another location for burial. But, she had returned the shovel and unless she bought one or borrowed one from someone else, she didn't bury Caylee.