> 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...

>you typically dont go and hangout on the DC floor, only the personel who is really required gets to go in.

times changed. I remember my friend bunch of years ago complaining to me that the DC at their branch of a large well known transnational was the favorite place of the employees for various informal activities, including that he was constantly dealing with used drug paraphernalia being left all over the place.

Answer to the comment below: yep, mostly syringes/needles.

Used drug paraphernalia? Were people shooting up or what? Surely smoking wouldn't be possible without alarms going off.

Usually there are muster stations where a designated emergency captain takes attendance for everyone who is expected to be present and a remote operations center that has access to HRIS data to know who is out/on vacation when that attendance is known.

Door locks during an emergency are usually denoted to be fail-open so someone doesn't get trapped, though that may differ depending on the security level of the facility.