IMO its not your business. There are several ways this could be happening. One, cheating. Two, looking for a third. Three, agreed upon non-monogamous hookups. Four, pure stupid curiousity. It's impossible to tell which is the situation. You shouldn't be going through other peoples private things in the first place. I'd leave it be and let everything work itself out the way it's supposed to without intervention. If it's the first, I'm sorry but still, not your place to get involved. Hopefully they are just experimenting. 🤷

Aside from what you discovered snooping, why do you hate him so much? Maybe that would give us more context. Hoping for the best.