Which locks the building down? I've lived in two buildings with a key fob entry system and a loss of power means the doors are locked until someone exits and props them open. Management didn't care since you could get out in case of a fire.
Unusually, the power went out at about 00:15. I got home at 00:30, and saw them in the courtyard. They'd taken the rubbish out, but couldn't get back into any part of the building with their RFID fob.
I have a fob plus actual keys.