My employer only allows self-certification up to 4 days absence.
I had a miscarriage several years ago and when I went back to work, one of the admin staff, who was responsible for the sick forms, saw me in the corridor and asked me why I'd been off so he could log it in his sickness recording system. I told him I wasn't prepared to discuss it with him in the corridor, and he could get it off my sick form once I had completed it. He insisted I tell him, I refused, we argued about it for a few minutes and then I walked away.
He was very apologetic once he'd got the completed form, but why don't people think?