X-13 stealth testing lab, technician terminal: Enter the door marked "Stealth Test Area 1", (blue glowing hexagon on floor). Head due East from the entrance around the upper catwalk to "Office #2 Observation Area" to the terminal in the north/east corner. Access entry #2 "Suggestion." and return to the Sink Central Intelligence Unit to upload the note and purchase the mod.
After buying the mod, you can go back to the X-13 research facility and download another schematic. You then can buy it again from The Sink.