The Soviets kept large amounts of captives for many years after the war. Von Luck was released in 1950, it appears, but there were roughly 30,000 who remained after him. These men were considered by the USSR not to be classified as POWs, but war criminals serving sentence for their crime, supposed to be 25 years. Some were guilty of some offense or other, but many likely were no more guilty than the average landser. That is neither here nor there though. The important thing is that in 1955/56, all remaining German prisoners were released as part of the establishment of diplomatic relations between West Germany and the USSR. Of course, some believed that there were more beyond that, who were undocumented and thus never released, but there is no solid proof of it.