In 1998 just after entering the Cape Town FIR from Johannesburg, heard a B727 freighter asking CT radar on th RT if they had an aircraft above them at 1 o'clock. CT replied that they were tracking unidentified traffic in the vacinity but without altitude information.
Shortly after this the B727 reported the "traffic" had disappeared.
CT replied that they were tracking it on radar at Mach 17....
