Never get involved with reading others' emails, documents, etc., that you are not required to be privy to.
Never ever let the temptation allow you to see others' performance reviews, salaries, politics. I've seen how it leads to telling someone else and then they become the go to person for information. And if the information is bad and they didn't share it, even though they had no idea, well, they didnt' say that there was a problem, the @$$#013! Hell, I've seen someone with access to the HR database pull up salaries of EVERYONE and share it out. "Oh, can you tell me how much Jason Mcboogerhead is making? What?!? I'm making $1k less?! WTF, time to march off to the manager!!!" [A manager who was stunned at the level of knowledge! AFAIK, no info was given out about how the salary info was found. I found out later when it was offered to me.]
Ignore any overheard conversations, it'll only be a couple of people talking, who knows the truth and what really is going on? You must throw out any info you "accidentally" pick up too. The obvious is the missing context of the info. As a manager, I've had other directors and managers openly talk about staffing, budget, bonuses, performance or lack thereof, in front of me. In all cases I threw away what I heard, after all, all I'm hearing is a snippet of a longer discussion. It's not my business to try to save John's job if he's pissed someone off, so I'm better off not worrying about it.
Sometimes I received a list of users to be locked out of their accounts. The only reasons to receive such a list is that they are being laid-off/let-go or in a heap of trouble. I never shared such a list with anyone. It was given to me, as a manager, in confidence. Keep that confidence. Even after the firing, I still didn't tell anyone, there's no point or net positive to be gained.
In another instance I was at a company that changed their HR such that you logged into a page, and it told you your salary, OT rates, etc. You could print your confirmation of employment for loans and such there too. But there was a bug. This bug allowed me to view everyone's salary, their bank account info and some other stuff in a nice neat chart. I immediately picked up the phone and called head office IT Security and talked them through the bug. They fixed it, phoned me back to test with me on the phone, thanked me and sent off a thank you cc'd to my manager, director, etc., praising my immediate response and "help" in fixing it.
What I didn't do was say, "Hey everybody, look at this!" and print it off, etc. Nor did I read further than a few lines and then remove it from my screen. To this day, I run into some of the higher-ups from then from time to time, they still remember me, who I was, only because of that email and that to them I was trustworthy.
It's not up to you to solve office politics, who said what to whom, or anything else. You are there to do IT. So do it and maintain your dignity and professionalism and just don't even think of looking.
You, and hopefully everyone else, will hopefully see that you are in a position of trust. You are trusted by many to keep secrets. If you can do that, it only helps your reputation. If someone can actually say you are trustworthy in your IT job then you've accomplished a lot and it only helps down the road when you want to switch jobs.