There are 4. It should be somewhere in the room floating around. Maybe one got bumped into a glitchy area. There was a save spot just outside the centrifuge room that you probably (should have) used. Try reloading and look again. The spot that frustrated me was after starting the centrifuge. Bloody annoying, a search only gave more confusing answers, and there's no objective direction marker. It's actually so stupid. Just leave the centrifuge where you entered. There's the save spot. Keep going through the second door and voila, mini cutscene and you're on your way again.
DUDE! your like the only person to reply on the planet! ! ! ! I ended up completing it on PC because I could not go any further on PS3!!! thanks for the info anyway hope it helps someone else though!
