Yeah Sobibor and the other extermination camps (Belzec and Treblinka) aren't really talked about in main stream Holocaust history. It's where they just straight up unloaded them off the trains and straight into the gas chambers. Then the camps were torn up and hidden before the war ended. They were fairly unknown for many years. At the end the prisoner laborers revolted at Sobibor and Treblinka and some escaped. The wikis on them are a fascinating read.