From where the fast travel station is located, go down the western set of stairs that lead into the northern area. When you enter the area, go west until you spot some more stairs leading downwards. Instead of using them, go on the nearby bridge and face south, looking up to spot the symbol in a small alcove. You can jump onto the ramp pillar nearby to reach it.
This one can be a bit tricky to get to, as it requires a bit of precise jumping. On your map, you should see a small path that leads to teh northcentral portion, which is called Helios Targeting Centrum. This isn't on the bottom level, but one above it and going south from that small path will bring you to three nets over a death pit. You need to jump to the third net and look up on the ceiling to find the symbol.
The first jump is a bit tricky to get right, as it's likely you will hit your head and not get enough elevation to land on it, so make sure you're sprinting to the very edge and activate your boost jump as soon as possible to reach it. The other two nets should be easy enough to reach without any troubles.
In the central section of the map (use the stairs on either side of the fast travel station), go to the eastern side and look for a long pipe that bends and curves, connecting to a wall on the north side. Follow this pipe all the way to where it connects to the wall to find the symbol.