Your best bet is to "forget" you read it; never acknowledge that you saw it, and assume the best.
For example, just because someone wrote about supposed "irregularities in the pension fund"; doesn't mean there are irregularities in the pension fund, it may just be some ignorant person spouting out / jumping to wrong conclusions.
Case to case basis. "irregularities in the pension fund" is something that could be ignored, "couldn't dispose of the corpse last night" puts you in a spot where you might be committing a crime by not reporting.
Actually, you'd probably be committing a crime by not reporting there too... In both cases, if it could be proven you were aware of it. What you're talking about is the different levels of moral responsibility between the two cases.
To answer the OP, as someone who's had root at large positions... Assuming you are not intentionally spying on something or doing something at the behest of a security directory, legal, or other internal affairs-ish agency (which probably doesn't exist at your smaller company), you should treat everything as if you were a cop and you didn't have a warrant. You're not going on a fishing expedition, but if something is "in plain view", it is not inappropriate to use common sense and reason to consider that information now available to you and make choices accordingly. If that means calling your CFO/Legal that's one thing, if it's police that's something else.
Overall, it's hard to go wrong with the time-tested advice sudo lectures you with, specifically #1/#3:
We trust you have received the usual lecture from the local System Administrator. It usually boils down to these three things:
#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.