Honestly, I don't think I'd mention you had been in his account. That would just result in getting the guilt off of your chest. Just STOP snooping around. That only adds fuel to your insecurity.
If you have an "open" marriage, he may think everything is fine with all this flirting. If you don't want an open marriage anymore, be prepared that he may not think this is fair. Good luck. I don't really get the concept of open marriage, but I am pretty old.