On the north end of the map, you’ll find a door that leads to an area with the exit to Research and Development. Facing that door, go to the right and you’ll find the symbol on the wall opposite the scanning machine used in the Voice Over optional mission.
Slightly south of the exit to Jack’s Office is a door you can enter that leads to a small area where Professor Nakayama is located. Note that you can access this after accepting An Urgent Message optional mission, which will lead you straight to this place. The symbol is on the back wall of the cell numbered TC-34527A.
This symbol is in the southeastern corner of the map, near the exit that leads to Veins of Helios. On the northern end of this little area, you should see a train car on the track and you want to between that and the north wall. The symbol will be on the actual track, on the ground by the north door. Note that you need to be on Watch Your Step or have finished it to access this area.