It turns out that he went for a swim and got out of the water when he had finished, but didn’t tell others before he went back to his accommodation.
As far as Ryboichuk was concerned, he’d just had a refreshing swim in Patong Bay. He was oblivious to the fact that his friends had reported him missing and rescuers had been trying to find him.
Authorities were alerted after Ryboichuk went to rent a car and the receptionist recognised the name in his passport. The two of them contacted the local lifeguard service to report that he was alive and well.
Source: https://www.phuketgazette.net/news/missing-ukranian-swimmer-turns-alive-well