I respect privacy although I know many do not. If I found documents or information that was not intended for me I would immediately leave it.
And it did happen where I received some documents from a company that I was bidding for a job. In addition to the bid requirements they also mistakenly gave me a copy of another contractor's bid. When I saw the letterhead I immediately returned the documents without reading further. I probably could have gotten a leg up on them, but I would have known throughout the entire job that I got it through unethical means. I don't want my company tainted like that.
Neither of us got the bid, by the way, But the company knew I was trying to be above board.
If it's my husband's text messages I would be terribly bored. I know all of his passwords. How? Because he asks me! "Honey, what's my iTunes password? Honey, do I have a Hotmail account?"
I've told him all of mine, too, and he remembers none of them.