Having successfully delivered the Windows 10 Creators Update, Microsoft engineers are onto their next project: writing the code that will make up “Redstone 3,” the next major update to Windows 10. But where's Windows 10 Mobile in all this?

Microsoft pushed Build 16170 of Windows 10 to the Fast Ring on Friday, an otherwise bland update that fixes a few bugs. The most important element, however, is what the build prepares for: adjusting the OneCore code underlying the PC, tablet, phone, IoT, Hololens and Xbox platforms so that new code can be checked in.

Though Microsoft hasn’t officially confirmed either a code name or a release date, it’s thought that the next update is being internally referred to as “Redstone 3,” and is due in the fall. Though we really have no idea what will be in Redstone 3, we can make some good guesses: Microsoft has already said that it had delayed the ”My People” experience until the fall release, and Microsoft has also implied that that’s when we'll see a more unified rollout for mixed reality—perhaps in conjunction with the Project Scorpio next-gen Xbox console.

For Insiders, the new build means one more thing: a return to bug infestations. We never recommend testing Insider versions of Windows 10 on production machines, but even development boxes can be plagued by buggy Insider code. (Microsoft did announce an Insider program for Business as part of the Build 16170 release, however, in case IT admins want to test out new code before it officially ships.) Consider changing your Insider tier to the Slow ring, or even Release Preview, to ensure the OS or apps don’t crash at an inopportune time.

For people who enjoy reading tea leaves, one other aspect of the new build is worth noting: the complete absence of Windows 10 Mobile. In the first build of the Creators Update, build 14901, Microsoft bent over backwards to reassure users that Mobile was being supported, and that new builds were forthcoming. Windows 10 Mobile devices will receive the Creators Update on April 25.

In the latest blog post, Mobile isn't mentioned at all, either in terms of bug fixes or new features. Was Microsoft's decision to sell a Microsoft-infused Android phone the real death knell for Windows 10 Mobile?

Microsoft says no. “We continue to develop Windows 10 Mobile and support Lumia phones such as the Lumia 650, Lumia 950, and Lumia 950 XL, and phones from OEM partners like Acer, Alcatel, HP, Trinity and VAIO, the company said in a statement. "Additionally, mobile and PC builds frequently ship at different times.”

Why this matters: Two things are going on here: One, Microsoft's moving quickly to Redstone 3, perhaps concerned that the fall deadline doesn't offer much wiggle room. The other, of course, is the limbo that currently imprisons Windows 10 Mobile. With a market share of less than 1 percent and a scant handful of available Windows phones, Windows 10 Mobile's viability is an open question. What we don't know is whether Microsoft's silence is an answer.

This story was updated at 12:40 PM on April 12 with a comment from Microsoft.