This week saw some reports coming through detailing that the Google Pixel 2 and Pixel 2 XL included a hidden menu button. This was first noted by Android Police and also confirmed in a later report by CNET. However, and while the button is present, and indeed acts as a menu key, it seems this is just a software glitch and one that Google intends to fix via a software update in the future.
The confirmation of the bug status was provided by Google to CNET following the initial reports. As such, those who have noticed the 'feature' on their devices (as well as those waiting on their pre-orders to be delivered) should expect the feature to be removed in due course. Although no firm information was provided in Google's response on when the fix will begin to roll out. One of the aspects that was first noted (and likely a clear indicator that it is a bug) was how it only seems to work in select apps. For example, the phone's Settings and Chrome app seem to be places where the menu button functionality is more evident. So this is not something that does work on all occasions, or when in most apps. In fact, it had also been noted working in a number of other apps including Calendar, Calculator, Docs, Messages, and YouTube. With the common denominator here being that these are all Google apps. Likely further evidence that this was a bug to begin with and not something that is a Google-specific feature.
As for how to get the feature to work, the navigation bar at the bottom of the screen is where it can be activated from. Although, with this being a hidden button, it is not one which is highlighted by any visual cues. Instead tapping the most right portion of the bar activates the feature along with two responses confirming the feature is working. Firstly, a tactile response will be noticed by the user (in the same way it would if a legit button is pressed) and secondly, through the opening of a menu in the top right corner of the screen. Keeping in mind that this only seems to be in effect on the new Pixel 2 and 2 XL, and is not understood to be working on the original Pixel and XL models.