> My guess for "how to make sure all people are out": typically in such industrial, high security facilities, each people going in would have to badge at each door, i.e. no holding the door for a group of people. Combined with many locked doors, this allows you to know exactly who is in which part of the building.

Holding doors was always a problem, but I guess that nowadays badges can have RFID or some other kind of wireless tracking so even if you hold the door, people in the control room can always know how many people are in each room, all the time. It's like 6 years now since the last time I was in a datacenter...