I don't know about DHS/TSA but at my Federal agency: Supervisors can require a doctor's note to substantiate sick leave of more than 3 consecutive days - though it's up to the supervisor's discretion (in my experience, typically only if you have a problem employee where you suspect leave abuse). As far as returning to work after having been sick, supervisors can require a doctor's clearance if the employee was seriously injured, had major surgery, or had been gravely ill - things where management wants to be sure the employee is physically able to work safely; or if the employee had been out with some highly contagious and dangerous disease.

I'd guess that DHS/TSA is somewhat similar since I think most agencies start with certain basic guidance from OPM (Office of Personnel Mgmt), tailored to meet agency specific needs, etc.