Small debate about this in our little office. Say colleague has had multiple unexplained absences during a particular month and the continuation of their employment depends on them certifying those illness / absences with a letter from a doctor.
If the colleague did bring in a doctor's letter which more or less in a prosaic manner outlines the illness etc. Could someone from HR contact the GP about the letter?
I'm thinking we couldn't because it would break confidentiality. But then again there's nothing in a simple conversation asking "Dr.X did you write this note about person Y?" That would break any confidentiality.