Found in Call of Pripyat, these are stashes with various loot, as well as Strelok's custom weapon.
There is a side mission, triggered by finding UAV wreckage and bringing black box to Nitro at Yanov station, however, these stashes CAN be looted WITHOUT triggering the mission.
Your main reward awaits at whichever stash you loot as the last, being Strelok's custom Sig 5K weapon.
SPOILERS OF THE SIDE MISSION
Stashes are at following locations:
1-Quarry, inisde of the bucket wheel excavator. Easiest way to get to this stash is aproaching from Substation workshop to the quarry. One can notice a tree close to the excavator. Climb the tree and jump from it on the excavator. Note that controller can sometimes spawn near this tree, and that there are chemical anomalies inisde the excavator's cabin. Stash is represented by a bag inside of the cabin.
2-Under Cement Factory. Approach from bloodsucker swamp (which, as the name suggests, contains three bloodsuckers). Follow water, and you should end up under the factory. After turning on the light/night vision, you should see a tunnel. Enter and follow it to the end. Stash is represented by a green-colored box. There are numerous hazards, ranging from bloodsuckers inside the swamp to the chemical anomalies both in the swamp and under factory, in front of tunnel entrance. Recommended is heading into swamp first. There is big log in the center. It is recommended to jump on it, since bloodsuckers can't get to you there, and throwing grenades/using shotgun to kill bloodsuckers as they will be coming towards you.
3-The very last stash is located at Jupiter factory. Once inside the factory follow the road . There is a stash on the left side of road, containing AKS74/u in full condition along with some ammo and medical supplies. There are a lot of dogs, so this gun will surely be useful. Following the road, it should be turning left. Follow the path to it's end. Once there, turn left and you should see a square like hole with water at the bottom and ladder. climb down the ladder (jumping down will hurt you a little bit, but you will survive unless at very low health). Note chemical anomalies in the area. The stash is represented by a pipe with a valve on it.