Stalker: Call of Pripyat → Quests → Lair of Bloodsuckers
We tell Boroda about the bloodsucker lair we've discovered. Boroda recognizes this as a serious threat and sends us to search for the poison gas. Perhaps Sych knows something. Sych does have information, but he's not willing to share it for free. It costs 2,000 rubles. Sych will tell us that a military convoy carrying gas cylinders once stopped nearby, near a bridge.
Find the convoy and the right truck, in the back of which you'll find a locked container containing a gas cylinder. In the truck's cabin is a document that will tell you where to find two keys to open the container.
-1 key - Key A - is in the safe of a car that's lagged behind the convoy. Head towards the forestry area; the car is halfway off the bridge, but still holding on.
Key 2 - Key B - is in the safe of the car that fell into the bridge gap. Go in the opposite direction from the forestry.
Once you've got all the keys, open the container and take the gas cylinder. Next, you need to find the ventilation shaft. It's marked on the map. Then, you need to install the gas cylinder in it, turn the valve, and wait 40 seconds for the gas to take effect.
During this time, two bloodsuckers will appear, and that's it. Or maybe none at all. Then you can go to Boroda and report the mission. You'll receive 5,000 rubles and a Veles detector as a reward.
Source: gamer.ru
See also:
Blood Moon Event (video)
Fallout Tactics: Random Encounters
Saturated Monohydric Alcohols