Mission: Camp elimination
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 dealt 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 has 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 an 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 Strelok's stash. There are 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 a hole in the wall, and a ladder. Jump onto the boxes and head up the ladder.
Mission: Find Strelok's group's stash
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 Strelok's 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.
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 staircase 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 Controller's sight. You don't have to kill the Controller so you can simply proceed forward to the ladder up. If you want to kill the Controller, run towards it until you up against and then shoot it a couple of times, a shotgun or AK will do. The only thing you gain is the satisfaction of having killed it, and not having to worry about it if you come this way again. However, you may wish to save fighting it for a later visit when you are better-equipped.