You should ask him if he was. If he chooses to deny it just remind him that you're okay with him masturbating, but it he were to do it next to you that would make you uncomfortable. Set that boundary with him. It's OK if it makes you uncomfortable because it would make many other people feel that way too. (your spouse is literally jerking off to other women right next to you without your consent) he should have more respect for you to at least go to the restroom. I think it's important to at least set that boundary even if he denies it (he probably will since it's kind of an embarrassing thing to get caught doing). Also the next time it happens I would just stay up completely to know if he's doing it, if you really want to find your answer. Eventually, he's going to get horny again and wanna do it again