As soon as you enter this area you'll be given a new optional mission, Eliminate the bandits by the Agroprom Research Institute, given by Sidorovich. Well, it's semi-optional because you'll need to get past the bandits that the mission require you to eliminate, to reach Streloks stash, so you might as well kill them and reap the reward later on.
Before you move on you may notice a dead stalker on the floor next to you. Loot him before you move on.
As you move forward from your starting position you should soon notice some stairs going down on the right side. The bandits are located in the room ahead - there are about 8 of them. You can sneak up on the corner of the doorway and use that for cover as you lean out and take down the bandits. They shouldn't pose too big a problem. Once you have defeated all of them the mission will be updated and you can now collect your reward; it's given by Sidorovich so remember to head back to him once you've finished the missions at the Agroprom Research Institute.
Once you've dealth with the bandits move west, down the spiral staircase, and through the tunnel at the bottom; the tunnel has a number of Fruit Punch anomalies but they shouldn't pose any real danger as long as you stay clear of them. At the end of the tunnel you'll find a long, curved room that also have a few anomalies in it, and more importantly also a Bloodsucker. You probably haven't met these nasty creatures before but they can be dangerous, especially because they can turn semi-invisible making them hard to detect and see.
Just opposite of the exit from the tunnel you should see some stairs going up to a ledge in the room. Move up there and along the ledge; at one point you should hear a roar, possibly from the Bloodsucker - the good news is that it doesn't mean that is has spotted you yet (I just think it's a predefined "event"). Continue further down the ledge and you should soon come upon a Urchin artifact. Pick it up and head back the way you came and into the main body of the room. Again, watch out for the Bloodsucker - once it notices you it's probably going to come charging at you.
Once you have the artifact and have dealt with the Bloodsucker, move on ahead. You'll come to a long, curved corridor with soldiers nearby. I have found that tossing a grenade at them, and then backing in the room (with the Bloodsucker) can be a good strategy; you can use the doorway as a choke point, and take down the soldiers as they pop out in front of you.
By now you should be able to see a marker on your minimap - that is the location of Streloks stash. There's two ways to reach it, either by going left or by going right. If you don't want to confront the soldiers (yet) you can try to sneak left, otherwise just take care of the soldiers and move towards the marker. Once you reach the marker you'll notice a couple of boxes, and hole in the wall, and a ladder. Jump onto the boxes and head up the ladder.
Once you've climbed the ladder and head into the room ahead, the Find Strelok's group's stash mission will be updated and you now need to search the stash for information about Strelok. Behind the board with the maps on it you should be able to find Streloks Flash Drive. Once again the mission will be updated and you now have to find a way out of the Agroprom Underground. Also, you'll get another mission: Find out what happened to Ghost.
The information on the flash drive will reveal that a guy named Fang has died, and that Strelok apparently went to the center of the Zone (alone), but it looks like a guy named Ghost knows exactly where Strelok has gone.
Strelok, I have sad news. Fang died. Couldn't meet you. Too bad you went to the center on your own. It was your choice though. I know where to search for you, and whom to ask. Ghost.
Before you leave the stash take your time to look over the place. You should be able to find a Fireball artifact and some other goodies, including a Bandit Jacket, an Akm 74/2U and a unique Akm 74/2 - the Fast-shooting Akm 74/2 which has an incredible firing rate. You should also be able to find some metal boxes containing ammunition, medkits and grenades. You can also find a Stalker Suit inside a wooden crate - if you face the exit to the stash, you can find the crate on the left side, behind the large pipe. Once you've ransacked the place it's time to move on.
When you're down again you may take the time to quickly move to the northeast - at the end of the long, curved tunnel you'll find a staricase with a Fruit Punch anomaly on it. Beneath the stairs you should be able to find a Gravi or Wrenched artifact.
You need to get out of the underground now. Simply follow the arrow on your minimap but be careful, there are more soldiers ahead once you reach a round room with stairs going up. Eliminate the soldiers as you progress. As you head down the final corridor leading to the marker on your minimap you will suddenly hear another loud roar - but this time it's not a Bloodsucker but instead another creature you haven't met before, namely the Controller. These guys are, in my opinion, pretty cool and have some very nice (and annoying) attacks. The primary attack of a Controller will "pull your mind out of your body" (or something like that) and leave you disoriented with a nice, blurring effect. Before he uses this attack you'll be given a warning in the form of a high-pitched tone that builds up; this gives you precious time to get out of the Controllers sight. You don't have to kill the Controller so you can simply proceed forward to the ladder up.
Before you move up you may wish to check the Diary of your PDA. I am still figuring out exactly where you find the information, but if you have been lucky you may have a map of the area you're heading into (along with soldier locations); this information will also reveal that there are fewer soldiers present at night, which makes this the easiest time to get through the area - I am not sure exactly when the numbers decrease so if you want to be sure it's "night time" I guess somewhere between 20 - 22 (8 - 10 PM) should do.
You're now back in the Agroprom Research Institute and when you enter this area you'll complete the Find Strelok's group's stash mission. The area ahead can be tricky because there are alot of soldiers around (about 12 or so), the nearest one being in a tower behind you (obscured by a tree). I have found that sprinting into the building ahead can be a good approach; hide there and wait for the soldiers to come to you. You can sit either in the hallway or inside the next room and simply take down the soldiers one by one as they approach. Once you have dealt with the majority of them, you can head out and take down the leftovers including those in the towers.
The documents you are looking for are located in the largest building in the area; the building has a siren on the roof that may already have been triggered. You can shoot the siren if you like. One problem about this area is that it spawns soldiers out of nowhere when the siren is on. The soldiers will spawn close to the eastern gate so watch your back if you're close and the siren is still active. I am not sure if shooting and disabling the siren on the roof prevent more soldiers from spawning, though.
Follow the northern wall towards the west and then move south along the road. There is a soldier in the tower ahead, so take care of him once you get close enough. Turn west and you should have the main building in sight, the one containing the documents you're after. On the southern side of the building you can find a window you can climb through - handy if you do not wish any unneeded confrontation with more soldiers.
You should be able to see the mission marker on your minimap now (if not, open your larger map to see where it is). On the second level, inside the large room, you can find a metal box with some ammunition in it. You need to head up to the third level - the icon on your minimap will change to a green cross once you're on the right one. You will be able to find the documents sitting on a shelf in a desk.
Message from Sidorovich.
Once you pick the documents the mission will be updated and you'll receive a message from Sidorovich telling you to go see the Barkeep at the Bar area. Please note that once you have the documents, another 6-8 soldiers will enter the area, so either slip out quietly or be ready for another fight. Anyways, deal with them as you see fit and make your way back to Garbage.
Once you've dealt with the bandits, now may be a good time to return to Sidorovich and collect your reward for the Camp elimination mission (from the Agroprom Underground). When you return to him he'll give you 1500 RU and two RGD-5 grenades. Once you come back to Garbage, you may receive another optional mission from him: Destroy the bandit camp at the Garbage. If you're tired of running back and forth by now I really don't blame you. You can decide to complete the mission if you wish - personally I didn't bother since I had just returned from Sidorovich. If you do however, you'll get another 1500 RU and two more RGD-5 grenades for your efforts.
Anyways whatever you decide to do you should finally make your way to the north, to the Bar. When you get close to the northern part of Garbage, you'll get a transmission from a Duty Warrant officer saying that they need help with defeating some mutants that are approaching. It's a pack of Boars moving in from the east. Once the mutants are dead you'll get another transmission that probably depends on how many mutants you managed to kill; even though the Warrant officer may say that you need to pay money to pass through the gate, this is actually not the case. When you get closer to him, he'll aim his shotgun at you and say something like "Buzz off stalker" - not really a friendly chap, is he? Although he may be sending different signals, go talk to him and he'll open the gate and let you through to the Bar area.