I found out while being in passthrough on my friends Oculus Quest. Its really cool to see how the controllers are built let alone my remotes around the house. Good for finding issues in Xbox Controllers if you have an internal break.
It is because the cameras "see" into the IR spectrum; in order to detect the controllers. First thing I did was use this new "feature" to check how much was left in my (black opaque) bottle of Hennessy Black. 😄
Technically the cameras are visible and IR spectrums, unlike the Rift CV1 cameras which are pure IR. The Quest/Rift-S need the visible spectrum too because indoor lighting gives off nearly zero IR, room tracking would fail. So they have a wider spectrum that covers IR and visible.
As long as they don't disable it now like Oneplus did with their monochrome sensor...
They can't disable IR on the existing Quest cameras, that requires installing a piece of IR filtering glass behind each lens (and that would stop touch controllers from working). It can't be ignored in software since it's just a monochrone signal (no distinction between IR and visible components). They can't turn the cameras off either since then there would be no tracking of the headset, controllers or hands.
The Oneplus could disable it because the IR camera was separate to the colour cameras, turn off the IR and the other 3 (normal, wide and telephoto) cameras still work like normal. On the Quest, all 4 cameras have IR.