@DiscoBeat yes, wells/crevices etc is my thinking.
He is almost certainly inland.
He almost certainly got hot/lost/confused, had a fall, was bitten by a snake or had a medical emergency
He is off the path (or they would have found him)
So he either wandered in the wrong direction or fell down something.
It's awful and I pray he is found, but I don't think it's a great mystery. If you consider the terrain, heat and all other factors, it's not a hugely shocking that he has come to harm. It's also possible his previous amnesia kicked in due to excessive heat and dehydration which may have caused him to become confused and walk in the wrong direction.
He seems to be a lovely man and I very much hope he is found alive today. He will need to have had a shit ton of water on him though and I fear the worst now.