That is not a clear-cut question.
If they decide he left his post, there is still the issue of whether he was captured and held prisoner. If so, once he was captured, he had no opportunity to return to his duty.
If he had run off, and come back 3 days later, he would be disciplined, but may or may not have been courtmarshalled, and likely would have been paid until he was adjucated and removed from service.
That didn’t happen since he was captured.
Good points, but I would question if he was in fact "captured". He left voluntarily, possibly to join them. Thus was he 'captured' or did he get involved with something that he later regretted?