Google has accidentally sent out a dogfood build for the November 2018 security patches to a Google Pixel owner. This update was pushed via OTA (over-the-air) and was discovered when the user woke up the next morning.
A dogfood build is a prerelease version of a software that developers use internally in a real life environment in order to perform quality assurance before it is released. As you can see from the notice below that was displayed to the user, these are confidential builds that are meant to be used internally by Google employees before they are released to the public.
Security update available CONFIDENTIAL INTERNAL ONLY -- This is a confidential Googlers-only OTA to update your devices to Android 9 with November 2018 security patches. You may use your device normally but do not discuss or comment on this update externally. Send feedback to android-dogfood-discuss@. see go/release-faqs for more info! Update size: 91.0 MB
According to a Reddit user, when they woke up, they discovered that the November 2018 security update was installed on their device and was waiting for a reboot to finalize the installation.
The user told BleepingComputer that they have no idea why they received the build and that they are not affiliated with Google.
"No I don't develop software or have any affiliation with Google whatsoever, the update just came through as any OTA would," the Reddit user MikiHaikonen told BleepingComputer. "The phone is completely stock (locked bootloader, no root/twrp, never used betas)."
BleepingComputer has contacted Google for comment, but had not heard back by the time of this publication.