In Gitta Sereny's book
Into that Darkness, Richard Glazar says this:
“Secrecy? Good heavens, there was no secrecy about Treblinka; all the Poles between there and Warsaw must have known about it, and lived off the proceeds. All the peasants came to barter, the Warsaw whores did business with the Ukrainians – it was a circus for all of them.”
Sereny also writes that "both Zabecki and Berek Rojzman had already spoken of the
peasants who tended their fields which adjoined the camp."
In a short typescript called "Plan of the Extermination Camp Treblinka - Execution," Zabecki also writes this:
Nevertheless, the camp had to be approached from the southeast, from the fields. The fields had been confiscated from their owners. Cultivation was prohibited. However, I noticed that farmers passed through or drove through this large, infertile area. [...] The camp's visibility was often obscured by clouds of smoke from burning corpses, enveloping the vicinity of the camp, depending on the wind direction. In this case, it was possible to get closer to the camp, as the sentries' visibility was limited.
So the ~100,000 people in the region (i.e., Christian Poles) knew about the death camp, came there to trade Jewish property, prostituted themselves to the Ukrainians (who were always drunk), and generally enjoyed the "circus" atmosphere of the camp, despite the choking clouds of corpse smoke that obscured the camp entirely, thereby ensuring its secrecy. Anytime a plane flew overhead,
Christmas trees (from the circus?) would be tossed on the corpses.