Tim Harford has a podcast series "Cautionary Tales" One of them ("Danger: Rocks Ahead") is on something called "Plan Continuation Bias" The idea is that someone makes a plan, Plan A, then something makes them tweak it slightly, then something else makes them tweak it slightly again. They think they're still following Plan A, but at some point there have been so many tweaks that Plan A is now an unplanned Plan B.
I think that's what has happened here, and why there is so much confusion. There was Plan A. Water taxi to the beach, spend the day there, water taxi back. MM either forgets his phone, or realises it needs charging so leaves it on charge. So his Plan A is slightly tweaked to a day without his phone, but that's a tiny tweak. It doesn't matter.
At the beach, after a swim, MM decides that he wants to head home, while the others stick to Plan A. If at that point he's intending to walk to Pedi, then get the bus, that's quite straightforward. So it's a second tweak, but it's a small one.
None of them quite realise that the combination of Tweak 1 (no phone) and Tweak 2 (head home alone) is cumulatively bigger than two small tweaks to the plan.
He gets to Pedi and for some reason doesn't get the bus, or decides to do some exploring before getting a later bus. Bus later, rather than bus now is a small tweak to the plan. But that's the third small tweak. He still thinks he's on Plan A. But the combination of tweak 1 (no phone) tweak 2 (head off alone) and tweak 3 (don't get the bus his wife thinks he's getting) means that he's now on unplanned Plan B, and his wife and friends have no idea.
So he ends up at the point of the last CCTV sighting. He might not even realise he's on the wrong path at this point. Or this route might be Tweak 4. He's now onto Plan B, which would require, perhaps, a map, and a bigger water supply, and proper hiking shoes. But he doesn't realise he's unequipped for Plan B, because he thinks he's still on Plan A, slightly tweaked.
"Plan Continuation Bias" It's a thing.