I was (legitimately) looking for a budget document in a shared folder at work, and opened a spreadsheet. As well as the budget info, it also contained a list of everyone's salaries, dating back to 2014 with forecasts to 2019.
'Everyone' really means that, including the CEO, and other senior folk. It also had details of bonuses and other stuff.
I can't un-see it and feel very bad that I know all the relativities and actual amounts. It's also in a document that anyone in the company could theoretically access.
WWYD? Tell someone? Ignore it? FWIW, we are not a very big company, about 50 people, not sure if that makes any difference.