If you don't exactly trust that your fiance did indeed let him out, and you know he'd had a lot of water during the day, I don't know that I'd stress one accident too horribly much. Did you see that he had awoken in it or was sleeping in it? Or was he just near it? Just getting at whether or not perhaps he had woken up without anyone noticing and peed before going back to sleep. In any case, based on your comments regarding your fiance, I wouldn't worry much unless it happens again. Could be kidney or urinary tract issues.