Jan Piwonski, a Polish railway worker at the Sobibor station, talking about the eerie silence following the extermination camp's first transport. No one knew where the Jews had gone, until the smell of their decomposing bodies spread through the area, and more transports kept arriving.
On the morning after the transport, when he had come back to work: "Simply silence. Nothing was happening … Apart from the sentries carrying out their normal guard duty, where we could see into the camp there were no groups visible doing any work. Already then, we began to suspect that the Jews must have been taken somewhere. But where?"
"After two or three days following the first transport, another transport arrived. At the same time, the mystery, let us say, of where the Jews from the first transport had gone began to reveal itself. In the area, the stench of decomposing human bodies began to spread."
"Very hot days had begun; it was the start of summer … around thirty degrees, and from what we observed and inferred, we concluded that the first transport had been murdered."
"When, after a few days, new transports began arriving, we knew that the people brought in those transports would be killed here."
Sourced from the Shoah outtakes, USHMM (RG-60.5031, reel FV3340). English captions translated from Piwonski's Polish and checked against the USHMM transcript, cut and captioned by me. This is condensed to avoid the interpreter interruptions, but I'll post the full clip as a reply.