After some tests, I find approximate boundaries of the zone where these drops occur (in my case). Approximate due to the fact that the drops aren't occur every mission load (except Pinemunde and H GDR30). Moreover, when I jump around the map using F11 and then wait for a while (like 10 minutes), the drops disappear altogether (until the mission is restarted).
In addition-while I was writing this message, I restarted the mission in order to check my theory about F11. After restart drops returns but when I did Alt+Tab to browser and back and they disappeared as well as after waiting after F11. Maybe it's just my PC issue (I mean drops disappearance after Alt+Tab/waiting), but it looks extremely strange.