It depends on the age of the grandparents, but there comes an age when they may start to develop cognitive issues such as dementia. The early stages will probably be almost unnoticeable but may be enough to make them less able to look after an active and curious child.
I haven't read all the sources for the case, but has there been any suggestion that Gus may be cognitively disabled or ND in some way? It seems to me strange that in an open and largely silent landscape he did not hear searchers calling him and return. It's possible that he was injured and unable to respond or that he managed to wander beyond hailing distance, but if he was cognitively disabled or ND he could have hunkered down or hidden somewhere and become trapped. From what I've seen, the homestead/station has a very small population so he should know everyone who would have gone out searching for him, at least in the early stages, and not therefore hidden from someone he didn't know.