His hat. I can't get past the idea that if some bird of prey, or some animal, snatched this child, his hat would be found nearby. If a 4 legged beast, there'd likely be blood, or a sign of some sort, drag marks, etc. A bird of prey? I just think him to be too sizable for such a scenario.
Gus either was taken from this property, or is on the property. He may have been murdered and buried somewhere in the outback. Or he decided to explore, and found himself in a place where he couldn't escape.
No footprints or other evidence pointing to his departure from the reported sand pile where he was reported to have last been seen is suspicious imho.