The true story was, that east of the island of Usedom was an aerial shooting range, know as "Luftschießzone 1 und 2". This was used by our Air Force and the Czechoslovakian Air Force. This shooting range was over open water and that's why western spy ships had the chance to monitoring this area all over the time.
IIRC it happend in the late 1960th or early 1970th. A german MiG-21 fired a R-3S missile (AA-2 Atoll), but the rocket engine failed and the missile falled down into the Baltic Sea. The West German spy ship was eyewitness. As soon as possible west german frogmen were flown by helicopter in that area with the task to search for the lost missile. At the same time the East German Kampfschwimmerkompanie KSK-17 got the same task. And so east and west german frogmen fought a race to find the missile. With a lucky end for the western side. Their frogmen found the missile, transported it to the spy ship and sent it home to West Germany.
And so the West Germans got a R-3S missile by first hand. Or with other words: shit happens.