In Norway you can lookup anyone's tax information. You can see who looks up your tax information, though, it is publicly available as long as you log on to government sites.

So basically everyone's salary from two years back is public information. It is often used by the press to check how much the richest people pay in taxes (or rather how badly they contribute by dodging taxes).

I've never heard of employers using it to check candidates' salaries, though. They usually just ask as part of the interview process.