Bob waits near a secured door, holding a box.

Bob waits near a secured door, holding a box. He waits until an employee walks up to the secured door and uses the special card in order to access the restricted area of the target company. Just as the employee opens the door, Bob walks up to the employee (still holding the box) and asks the employee to hold the door open so that he can enter. What is the best way to undermine the social engineering activity of tailgating?