This is slightly off-topic.
A few years ago on RR, there was a lively (read heavy flamage) thread on using confidential information that came to one's attention in the course of a gig.
Now, it's definitely bad, IMO, to disclose or use a client's data, like credit cards. In fact, it could be criminal.
OTOH, and this was my example, it is good business to use information about your own contract, even if you came upon it accidentally. As I was retreiving copy from the network printer, a page with my name in caps was on top of the heap, spelling out my rate (I knew that already), and also the exact amount of my budgeted hours and the fact that there would be no extension.
Well, I was roundly flamed on RR for admitting to reading that. I was supposed to ignore it even though it directly concened me. My counter was that if client was serious about my not seeing it, they would have taken steps to hide it or at least not leave it lying in a public place. And since it had my name on it, I would have to read enough to see in what way it did or did not "concern" me.
A lot of confidential information gets disclosed in just that way. Documents lying about in a common area near faxes and printers are just waiting for "unauthorized" reading, but if it's just lying there, who's to stop you?