In secure facilities (particularly defense related) where secret information or networks are in use, visitors must be accompanied by an escort and announced before entering. Everyone has to stop any actual sensitive work, shut off their monitors to sensitive systems, and cover up sensitive documents on your desk. Then as long as the visitor is in a certain area of the building, red flashing lights (no siren/noise) must stay on to alert everyone else that an un-cleared visitor is currently in the area. This is why most "actual work" gets done in basements and extra-secure rooms where visitors rarely ever see, because the constant interruptions are devastating.