Any marks on him, OP? Whatever he was washing off, looks like it was from the outside... I'm thinking blood.
Check out his hands, face, neck for broken skin or bruising. Best case, he got in a punch up. Worst case, he's done serious harm to someone -raped someone or worse.
Have a sneaky peek into inside and outside bins and see if anything has been rammed down inside - potential weapon, other clothing etc.
If at any point he asks you to cover for him or starts saying he was actually home by 11pm and you were sound asleep, be alert.
Is he capable of being violent, especially when drunk? Check local news for police appeals for witnesses etc. Eyes and ears open, moth shut, I think. Your phone has evidence of your calls; don't leave it lying around. Put a password on it of you don't have one already.
Everyone always says, "Surely his wife must have known?!" Nup.
I hope it's nothing but a selfish night out. It's probably nothing. But it could be something.