I was thinking about this at the negotiation stage, before hiring occurs. I can't imagine that an employer would try this after the hiring stage.

As for salary information, there are other ways to know how much someone else is making without accessing "confidential information". You can find out socially (e.g. from them), knowing their performance combined with position, etc.

EDIT: It is worth noting that these are facts I consider when I am interviewing for a new job, not something I put into practice. I wouldn't tell my current employer this information about a perspective hire, nor would I as an employer ask for this information.