According to a new report shared by XDA Developers, Google seems to be testing a new piece of software on the Google Pixelbook, called "AltOS". The Google Pixelbook is basically the main devices for testing Fuchsia OS, along with booting various Linux apps, and now AltOS seems to be a part of the equation as well. AltOS was spotted by a Reddit user called "-nbsp-", on the ChromeOS subreddit, and it is referred to as a new mode for the Google Pixelbook.
Now, a few commits related to this mode have been spotted, and it's rather unclear as to what AltOS is, though its name suggests that it could be a mode to choose an "Alternative OS (AltOS)", which would apply if two operating systems (or more) were installed on a single device. It is worth noting that the separation between Chrome OS and AltOS mode is clearly noted in the commits that popped up. So, AltOS is probably not a brand new operating system, or anything of the sort, but just a mode which will perform a certain OS-related function, such as switch between two OS'. It's unlikely that this is a new OS because Google is already working on Fuchsia OS, not to mention that Chrome OS and Android (not to mention Wear OS) are both Google's operating systems, so making a brand new operating system simply does not make sense.
This is the first time AltOS was spotted, and chances are we'll see it more and more from this point on, and the next time it pops up, it will hopefully reveal more info, as we're in the dark at the moment. It is even possible that Google will talk about AltOS during this year's Google Developers Conference (Google I/O), which is set to take place soon, At Google I/O, the Mountain View giant shares a ton of info with developers, and talks very little about new products directly, at least that was the case in the last couple of years. The company will talk about Android P for sure, the same can be said about Machine Learning, but AltOS may be a part of the show as well, we'll see.