This is an Android/Daydream notification. The same thing happens when you take a screenshot in Chrome or Daydream Home. Strangely, this does not appear on a different Pixel XL running the same version of Android and Google VR Services.

After some investigation, we determined that this is related to the system Do not disturb (DnD) setting while in VR. By default, the phone is in DnD when in VR regardless of its setting outside VR. It appears that this specific device was somehow switched from the default in-VR DnD level.

To reproduce:
(1) After entering VR and with the phone NOT in the headset, pull down the system tray with your finger.
(2) Click the DnD icon.
(3) Select "Priority only."
(4) Repeat the steps in the original description.