I've been there, and I've done that.
I did it when I moved house a few months after we broke up, but were still running a business together. I was switching computers for the move and whilst I was transferring data, I saw that he had a ***** account that I didn't know about. Not that it was any of my business, however it intrigued me because he is a technoidiot. I should say at this point that he was harrassing me continually about us getting back together, including trying to get me to go on holiday with him to Jamaica, which is where he was at that time. As I had access to his work email account (for legit purposes), I did the "forgotten password?" thing and got access to his ***** account.
What I found both horrified and delighted me. I found out he was not only on holiday with someone (what would he have done if I'd said yes when he asked me to go a day before he left?!!) but he was playing her off against 3 other women. Not only that, but he was using the EXACT same words and MO to all of them that he used with me. I left the house the next day with a sense of validation that I had never been able to get from him. I needed to know that it wasn't about me and I got all the proof I needed right there. I subsequently got more proof than I ever wanted later, but that's another story.
I am not proud of what I did. I massively invaded his privacy, although in mitigation that evening I did tell him what I had done (when he made his nightly call from Jamaica to tell me how much he missed me),and told him to change his password. I have no idea whether he did, because I never checked that account again. And I will never again check email, phone, bills or anything that are the property of someone else without their express permission. My gut tells me all I need to know. I get enough proof in actions.
The thing that makes my story somewhat different to yours, though, is that I had no idea what was going on prior to me checking. Some might say that I didn't need to check anything, given that I had left. And that is a fair point. Had he not been bombarding me with emails/texts/phone calls/flowers/messages behind planes (well, not quite!), then I would not have done it. However, I guess I needed something extra to validate my decision and to add some strength to my "No". Also, I think I was still reeling from the chaos of the previous few years and the fact that so much was ethereal, rather than concrete, and I wanted some hard proof that I had made the right decision.
I got what I needed. It wasn't I really wanted (i.e. to be thought of as special), however it was what helped me move on. You already have much of this information, so why do you need more? Do you think it might be the whole drama thing? Or are you just being plain nosy? LOL!