In fairness, you’re talking about this happening in the midst of the greatest displacement of people in history. The vast majority of the area’s pre-war residents, Germans, were forced out* and large areas weren’t repopulated for years. In those circumstances it’s not inconceivable that a project to hide something on this scale could have succeeded – tracks laid/moved/torn up, forest/field replanted…
*Or had died – 40,000 civilians died at Breslau.