A test build of the May 2017 security patch has reportedly been sent out to some Pixel XL owners on accident, alerting them that a new software update was available to download and install to their devices. The size of the software was just over 62MB in file size and based on the information in the screenshot below it clearly states that the software update was for the security patch that is meant to hit devices in May. It's not abnormal for Google to send out software earlier than usual, but that isn't the case with this particular software as it also clearly states that the update is meant for internal use only and that the details are "confidential."
Normally this might excite some users if it was actually a new build of software that was meant to include new features and other improvements to the operating system, but this is just a security patch and while it would definitely be useful to have a leg up on any of the latest security issues that might need to be fixed well before other users, just the nature of this software being for internal use only could mean bad news for anyone who decided to install it as it likely isn't ready for public consumption, which is a probably a big reason why this wasn't supposed to be sent out to end users.
How this happened isn't clear, but what's more important than the how is that it simply happened. There isn't any information about the update included in the screenshot but if users who received it do decide to install the software it probably won't be long before someone makes the information available to the rest of the public which is what probably what Google doesn't want, and that could be bad for Google if the software has problems. This only seems to be showing up on Pixel XL devices as the update alert does note that it's meant for a Pixel XL, which could mean that Pixel versions of the software were better kept under wraps or they simply weren't being tested. In either case, anyone who is wishing they had this update won't have to wait too long as it should be showing up for people normally within the first week of May.