One day, just after I got in the elevator and hit the button, one of the building's maintenance workers hurried in. He got out his huge keyring, and stuck a key into the elevator's control panel. I heard a chime, like a floor had been selected, but no new numbers lit up.
The doors closed, the car rose for just a few seconds, and then stopped. The doors opened, and a level I had never seen before appeared before me.
It turns out that this building had a secret level, between the lobby level and the 2nd floor, containing the HVAC controls and the maintenance workshop.