[You wouldn't build a death camp near towns with the interior easily viewed.  You would place it inside an off limits military base, or in a remote location such as in the middle of a forest like the Katyn massacre.  One mile from a town?  Easily viewed from outside like Treblinka?]

Quotes
Much like Treblinka, the Belzec camp could easily be looked into from the nearby rail line and road. The town of Belzec was located about 1 mile north of the camp....The Majdanek camp is located at the outskirts of the city of Lublin. Just as for Treblinka, the surrounding fields were cultivated right up to the camp boundary. The alleged gas chambers and the crematorium were outside the camp proper, openly visible and accessible to thousands of people living in the suburbs of Lublin. AIR PHOTO EVIDENCE by JOHN CLIVE BALL

After the war the Allies said no mass-murders occurred at walled-camps like Mauthausen, yet the mass-murder of thousands-a-day were kept secret for over one year in camps like Auschwitz, Treblinka and Majdanek, surrounded by see-through wire-fences, roads, and towns, where it would have been impossible to keep mass-murders secret for even one day.
    In 1945 Soviet propagandists made a mistake by alleging secret mass-murders at visible camps like Auschwitz and Treblinka, instead of at camps like Mauthausen where the walls would have allowed secrecy. Years later when they realized their mistake it was too late to change as Auschwitz had been promoted as the largest, most important mass-murder camp. http://www.air-photo.com/english/

In 1942-1943, the 'extermination camp' area was practically devoid of trees or large shrubbery. As a result, the neighboring farm folk and passers-by could easily observe, through the barbed-wire fence, the prisoners and the guards as well as the various buildings of a camp that is now said to have been ultra-secret. From the perspective of someone facing the entrance to the camp, the Olszuk family farm was located a mile and a quarter to the left, while their plot lay, to the immediate right, 300 meters from the camp's eastern limit. Thus, Marian Olszuk passed close by the 'extermination camp' every day that he went to work at the quarry, and when he worked on the family plot, he was also right near the 'extermination camp.'...Even though, of course, he never entered the camp area, every day people gathered in groups outside the front gate, openly engaging in barter and black market dealing. ....Had Marian Olszuk ever noticed signs of homicidal activities by the Germans in this 'extermination camp?' His answer was No.
....Remarkably, after the 'liberation' of Poland and after the war, no administrative or police authority questioned him about what had taken place at Treblinka. After the war there were official commissions of inquiry, which issued extravagant reports, comparable to the Soviet report on Katyn (USSR-008). But none of those commissions ever asked the Olszuks to testify. All the same, the official camp guide, Marja Pisarek, coldly asserted in 1988 that "No one in the vicinity will talk to you". But Marian Olszuk, obviously, was able and willing to talk to us at length, and, unlike another Polish witness, clear-headedly. [1988] Treblinka: An Exceptional

Birkenau 'gas chamber'