Trapper offers this quest after the player has completed Hunting: Kill the Swamp Bloodsuckers. It seems a group of stalkers were chased out of a station by some unknown mutants. Trapper asks you to investigate. The place to visit is the eastern tunnels near the Bitumen anomaly. A door to the east of the station platform is the way in. After a seemingly fruitless stair climb and hallway crawl, the corridors open up into an indoor trainyard. Night vision is handy here. The 'unidentified mutants' are actually Burers on the ground floor (the grunting is usually a dead giveaway).