The cams would not turn on, or would shut off, when a person got too far away, failing to detect movement. We were told that by LE, and also see that in the video we've been provided. There were also areas with no cam coverage. And the definition of "southwest corner of the interior" is somewhat vague, as "interior" could be indicating either the auditorium (which was the interior portion of the building) or the entire building, and "southwest corner" could be a small area or the entire southwest portion.
Here's a map working within those vague terms, and then showing some possible ways that MB could have been seen entering until she went towards where the perp was and lost sight of by cams, then was killed here or there, and the killer leave without being picked up on cam. The orange lines define the possible limits of "southwest corner" and the red X's show places her body may have been found, where perp still could have left without being seen by cam.