From the entrance and the fast travel station, go east and north until you reach the Stalker Biome area. Instead of progressing any further, go west from the checkpoint here and look on the southern wall to find the symbol.
From the entrance, go down the central path, which will lead you to the Aquatic Observation Area. There should be a doorway on the east side that leads to the Aquatic Life Stabilization Area. Go south and down the ramp, then turn around and look behind the ammo crate in the little alcove to find the symbol on the wall.
Go to the southeast corner of the map, which is where the X-STLK-23 spawns during Science and Violence (as well as afterwards). From the platform it spawns on, look north and slightly west to spot the symbol on a wall, near the small alcove.