There are stories, mostly unfounded, about thousands of American (but no British) POW/MIAs who never returned after liberation by the Soviets:
Remember Stalin and the West were allies until soon after WWII. At the Yalta meeting between Churchill, Stalin and Roosevelt, the plight of POWs was discussed; as the camps were captured the POWs were to be repatriated, and the majority were certainly handed over - although in some cases it took a long time to be returned.
If any remained behind it was for one of the following reasons:
They were too ill to travel - this would have affected a sizable percentage of POWs as wounds or disease made travel impossible.
They became dedicated Communists - although I have never read of any British or American troops doing this - there were many German POWs who became communists, notably Paulus, the German commander at Stalingrad.
They had committed crimes after the camps were liberated. They would have been held until MPs from the POWs country could arrange for them to be returned.
They possessed useful knowledge - i.e the crew of the B-29s that crashed, and were either too valuable for the Soviets to release, or had died under interrogation.
They were so hostile to the Soviets that they were taken away and "re-educated".
There are persistent internet rumours of thousands of American MIAs who should have been liberated by the Soviets and who never returned home, but, I can find no reliable source for the information, and no scholarly article. I suspect that the numbers have been exaggerated (and may only be in the hundreds - notably the crew of the B-29s that crashed on Soviet soil) and that, in reality, the majority of American MIAs were those who were too ill to travel, or who died of their wounds or disease and who's records got lost / confused due to the fog of war,
See:
http://www.loc.gov/exhibits/archives/wora.html