The real mystery of Auschwitz is why it would ever have been used as an extermination camp in the first place if it ever was. The problem is specifically that prior to the construction of the four huge crematoria at Auschwitz which would only exterminate about a million people, Treblinka was supposedly in operation for a year. And although Treblinka was supposedly studied prior to the construction of these crematoria, Treblinka had managed to exterminate one million people using only five small buildings and burning the bodies in an open pit.
So the mystery is, why the construction costs at Auschwitz when a simple hole in the ground was known to have worked? Part of it is due to the story having changed over the years. Up until a few years ago, Auschwitz was supposed to have exterminated four and one half million people. Now that it is down to barely one million the huge extra expense makes not the least sense.
Another part of the mystery is apparent if one assumes there was an attempt to keep the extermination a secret. Treblinka was small and out of the way. Auschwitz was not only large and in the open but the inmates were regularly circulating in the local towns and local craftsmen were regularly working inside the camp.
Under these circumstances it would have been impossible to keep a secret at Auschwitz and much, much easier at Treblinka. Yet Treblinka was shut down in favor on a more expensive location where it was impossible to keep a secret.