I don't think him not having his phone with him is odd, wouldn't take mine on a trip to the beach, take as little as possible and nothing I'd worry about losing, but then deciding to go on a mountain walk without it or any other navigational tool, or stopping to get a drink, in the heat of the day is odd.
The walk back to the town, which he did successfully fine, the rest is not, I don't think, something an intelligent, experienced person thinking rationally would do.