The expulsion to the East reported above was an ongoing affair. After Austria was annexed to Germany in March 1938, the Polish government was afraid that the approximately 20,000 Austrian Jews with Polish citizenship would flee back to Poland. It thus suspended the validity of all Polish passports whose holders had been abroad for more than five years. This law took effect on 31 March 1938, but it was only after the Munich agreement that a decree was issued allowing the passports of all Poles living abroad to be checked. After 31 October 1938 the holders of Polish passports issued abroad were only allowed on to Polish territory if their passports contained a special note made by Polish consulates. In this way, most of the 50,000 Polish Jews settled in Germany would lose their state citizenship overnight.
After German-Polish talks failed, the German Foreign Ministry handed over the whole affair to the Gestapo, which on 27 October 1938 started forcibly deporting Polish Jews over the Polish border
. In some places only the men were deported, since the Nazis expected they would be joined by their wives and children all the same, while in other places women and children were deported as well. Those arrested included old people, some of whom died during deportation. There were also suicides
. The arrested Jews were forced, through threats and violence, to illegally cross the border with Poland
. In all, approximately 17,000 people were expelled in this way from Germany. However, the Polish authorities refused to accept them, and so most of them had to live for many long weeks in no man's land, or the Polish border area. In most cases they were driven into the surroundings of the Polish towns of Zbaszyn and Bytom. In Zbaszyn, according to various sources, between six and ten thousand Jews gathered in the space of a few days. A large refugee camp was created in Zbaszyn, with help from Jewish aid organisations. From November 1938 to August 1939 Zbąszyń housed a transit camp for Jews expelled from Germany during the so-called Polenaktion (27–29 October 1938). These Js were transported to the Polish border on 29 October—about 8,000–9,000 to Zbąszyń, 5,000–6,000 to Bytom, and the rest to Chojnice and Gdynia. The deportees were driven by armed German guards with dogs on a cold and rainy night. Many had to cross the border on foot; some died and others were later hospitalized.The personal freedom of the refugees was restricted. It was not until the end of November 1938 that the Polish authorities decided to disband the camp and allow the refugees residency in Poland. With the help of Jewish communities and organisations, many of them managed to arrange travel visas and to leave the country, or to settle in Poland. After talks with the Polish authorities, the Nazis allowed the temporary return of a small group of men so that they could put their affairs in order in Germany. Finally, the Polish authorities also permitted the arrival of the family members of Jews expelled at the end of October 1938.
This pattern of expulsion continued after the Polish invasion, especially as the Gestapo had set a methodology of "best practice" in place; if it worked in the past then it would work in the future, especially as many of the same men were doing the job of eviction.
Once the Molotov demarcation line (border) was established eviction camps were set up to evict Js into Polish Soviet territory.
As mentioned these Js left with the Russian army and considered undesirables due to Western influence.
Whether or not the transit camps were indeed Treblinka, Sobibor and Belzec is another matter but at the moment most likely.
According to wiki
The Expulsion of Poles by Nazi Germany during World War II was a massive Nazi German operation consisting of the forced resettlement of over 1.7 million Poles from all territories of occupied Poland with the aim of their geopolitical Germanization.
. Js may have been given special preference for expulsion.