So he and his wife were staying with friends in or near the town of Symi.
I think that I read, or it was implied that they took a boat to a beach.
Michael Mosley then decided to walk back rather than return by boat so left his wife on the beach.
It’s not clear whether he forgot his phone on the beach and his wife brought it back to the friends house or whether he’d left his phone at the house in the morning.
He must have taken the coastal path to Pedi to start with (a 20 minute walk) as that was the only possible route.
He’s seen on CCTV in Pedi half an hour later. His options then would have been:
A short walk to Symi through a built up area.
A boat trip to Symi from the marina in Pedi.
A much longer walk around the coast.
Returning to the beach where his wife was.
If he had travelled from Symi to the beach by boat in the morning then he may have felt more confident continuing on the coastal route not realising that the shorter route was away from the sea.
It looks like the coast from Pedi to Symi is much longer than the beach to Pedi stretch