Google mistakenly releases confidential test build of next Android security update to Pixel XL
Android Police reports that some Pixel XL owners received a "Dogfood" test build of the planned security patch for May 2017. The OTA notification specifically reads that this is a "CONFIDENTIAL INTERNAL ONLY" update which Google employees are supposed to test out, but are not allowed to "discuss or comment" externally.
Weighing in at 62 MB, the patch doesn't seem to bring any visible changes or new features. One user says that he got the new Play Store UI after updating, but considering that this is an isolated case, it's likely that this was caused by an accidental server-side change.
source: Android Police