I've been in that position of having to snoop. It's not a good feeling, but something within the relationship, your inner voice, your gut feeling, is causing you to do so. A totally committed, secure relationship should never drive you to have to snoop. Something wasn't / isn't right, and you sense it. Trust your instincts. Don't berate yourself from having snooped. Not sure if you should confront him, as this will put the onus on you, and his reaction will probably be that your are at fault.

Despite talk of marriage, there is no need for you to remain in this relationship if it's making you feel insecure. You don't even have to give much of an explanation, though it may seem that you do. Just walk away.