In my experience, they don't shoot at you if you are in cover.
They have to see you to fire the first volley of three rockets. Then they fire the second volley whether they see you or not, for a total of 6 rockets. In the original Doom 2, they fired three volleys of two rockets, also for a total of 6.
Incidentally, Cyberdemons in original Doom behaved the same way... Once they see you and start firing, they fire off three rockets even if you go out of their view while they're firing. It's especially funny if you get them to fight something small, like a Lost Soul, which gets pulverized by the first rocket, but Cybie keeps firing to get his total of three rockets anyway. :)
EDIT: As for the thread subject, I also have no idea how to pick up DS. My latest theory was apparently incorrect, and the latest clues that someone gave me don't make sense to me. I understand that the DS is from a manga I'm not familiar with called "Berserk" and it's a big-ass sword and I guess that's supposed to be a clue, but I haven't been able to make any sense of it so far. :|