This is a problem with passive security as well. How do you NOT NOTICE guys rapelling down a wall? Well, if you have no eyes assigned to watch that wall, because you are certain your passive means will work.
It actually ties into the shooting at the school, because the school has passive methods of protection — laws making things illegal, locks on doors, lockdown procedures. No active security measures, like a person who can repel a gunman.
Maybe they dropped out the sheets as a ruse and bribed a guard to let them out a door.