I really agree that when we feel we must stoop to looking at dirty laundry for answers as to who our partner is, this is a pretty big indication that the relationship is not salvageable. I'd look for symptoms in your relationship indicative of someone who is distracted elsewhere verses checking out their dirty underwear. good luck
I am not sure if anyone can help you out with this.
He told you what happened and unless you have information that proves otherwise then that's that.............. in other words none of this points to cheating if that's your concern.
Are you having problems in your marriage? Checking someone's underwear really isn't a good sign your marriage is in a good place.