Mark Gurman, Bloomberg (via Dan Moren):
The Cupertino, California-based technology giant never disclosed this component and the device currently lacks consumer-facing features that use it. The company has internally discussed using the sensor to determine a room’s temperature and humidity so internet-connected thermostats can adjust different parts of a home based on current conditions, according to people familiar with the situation. The hardware could also let the HomePod mini automatically trigger other actions, say turning a fan on or off, depending on the temperature.
As far as hidden components go, this is much better than finding out your thermostat has a microphone.