This is a real story from my previous company: One day in our 1,000+ person office, I was sitting on the fourth-floor petting one of the office dogs when the windows went dark. A massive cloud of black smoke had surrounded the building. None of the fire alarms went off, and people were unsure what to do, so we alternated between packing up and deciding to stay inside where it was not smoky and dark. Suddenly the floor began to fill with the nauseating smell of burning rubber, and people started coughing and heading for the exits. As we reached the exit, most of the office was crowding the stairwells to escape the toxic smell. The first floor was visibly smokey, and it was steadily rising through the building. It had reached the sixth floor before the alarms had gone off, and facilities mandated an evacuation of the building.
It turns out that down the street, a large patch of astroturf had caught fire at a food truck park. About half an acre had burned before it was extinguished.
Had we installed workplace sensors downstairs, we could have safely evacuated the offices with time before the smoke overwhelmed all levels of the building and panic ensued.