When social security disability contacts the past employers of a person applying for, or applying to renew (via continuing disability review) their social security disability eligibility, who finds out about the applicant's medical conditions? Do coworkers find out? Does your boss find out?

I am asking because privacy can be important for many reasons including references and letters of recommendation. I believe that some key people might lose faith in the ability of an employee who they found to be on disability with significant medical problems even if that employee did good work.