Below are two graphics that the American Federation of Government Employees (AFGE), a labor union which represents most Social Security employees, sent to its members recently. Apparently, the milk carton graphic was sent 25 days after the "Where is Saul" graphic.
I can guess that Social Security has some online system that employees use to sign in and out and that any agency employees can check to see if any other agency employee is on duty. If that's the case, I don't know why Saul wouldn't use it. It seems quite unlikely that he was away from his office for more than two months. I hope that's not the case.