If they aren't in physical danger why would they leave the office before 6pm?

Has to be an extreme one-off case, but not completely implausible. Self reported overtime pay and cramped condominiums incentivize especially white-collar workers to slack and stay, and strong economy traditionally meant weaker incentives for management to control it(before factoring in that middle managers themselves might have slow cooking divorce too).

Nowadays many organizations have overtime pay caps, monitoring, and enforcement/encouragements - e.g. MDM on laptops, SSO login histories, or by turning off lights and HVAC. It is improving at a rate and in that kind of spirit, perhaps just not using glass breaking drones.