Looking into the family history of a friend of a friend, I have brief details of two events for her mother, both of them from a birth certificate.
The mother (surname Stickel) was born in Warka (presumably Poland) in 1935; and gave birth to an illegitimate daughter in Himmelsthür (Germany) in 1953.
One possible explanation is that she was Volksdeutsche, and fled or was expelled from Poland in the aftermath of World War 2. She was too young before 1945 to be a forced labourer who chose not to return to Poland when it was under Russian control? Or is there an explanation I haven't thought of? I suppose it's also possible that she was a Jewish Displaced Person who chose to settle in West Germany.
How can I explore these possibilities further? Is there a 'Polish' equivalent of the sources described at Finding Heimatvertriebene in Germany?