I searched the article for "occupancy" and nothing. These ultrasonic tones are most likely ultrasonic occupancy sensors used for lighting and/or security. Typically they're 25kHz or 40 kHz (i.e. out of normal human hearing range), but it's possible that they are measuring some subharmonics of the sensors. I've certainly encountered ones that have mechanical issues and will spit out frequencies that I can hear too well.