The second is the various sources that have been referenced which suggest that groups of Jews that are supposed to have been gassed were later seen at locations further east. These sources (those that we know of so far, at least) are more limited than one would like, but they are there.
The third, which hasn't been mentioned here yet, is that Himmler called Sobibor a transit camp. There are three letters concerning this:
In
the first, Himmler orders transit camp Sobibor converted into a concentration camp so that a depot for the handling of captured ammunition can be set up there.
Oswald Pohl, after consulting Globocnik (both of these men had authority over Sobibor)
responded and suggested that there is no need to convert transit camp Sobibor into a concentration camp, because it will be possible to set up a depot for the handling of captured ammunition without making such a conversion. Himmler
replied and approved this plan. The links above show these documents in the English translation from Nuremberg; however the translation wrongly renders the German
Durchgangslager as "transient camp", when it should be "transit camp".
The revisionist view, or course, is that Himmler and Pohl called Sobibor a transit camp because it was a transit camp of some sort.
One orthodox view is that Himmler's use of "transit camp" is an example of "coded language" along the lines of "transit camp to six feet under, ha ha." Another view, sometimes advanced by Raul Hilberg, is that the German leadership invented "coded" terms like "transit camp" to describe extermination in order to protect themselves from thinking about their actions; on this view Himmler called Sobibor a transit camp because he didn't like to think about exterminating the Jews, even though he supposedly played a central role in ordering and implementing their extermination. Finally, I have seen suggestions that Himmler called Sobibor a transit camp because he just didn't know what it was, which seems pretty far fetched give that he is supposed to have played a central role in planning the extermination of the Jews. On this view the reason that Pohl also describes Sobibor as a transit camp is that he was keeping the fact that Sobibor was an extermination camp secret from Himmler.