There is a new Authenticator app for Windows 10 that Microsoft is now testing internally, and it was recently spotted by Twitter user WalkingCat in the Windows Store, under the name of "Phone Sign-in Beta". You can access the page but it is not yet available for download.
The description of the app shows that it will allow Windows 10 Mobile users to unlock a Windows 10 PC over Bluetooth, by simply tapping on a nearby PC from a list of synced devices.
With Windows 10, Microsoft has already introduced new secure ways to sign into your PC without a password, but the problem is that a lot of Windows 10 devices simply lack the hardware support for features such as Windows Hello, so this app would offer a solution for quite a few people, until the hardware becomes more ubiquitous.
From our testing, the app can only be downloaded on Lumia phones running Insider Preview build 14267, so it's probably safe to assume that it will arrive alongside Redstone. While sign-in options are currently limited to work and school accounts, the app description notes the following:
Future versions will include support for Microsoft account, a sign-in solution for browsers and VPN, one-time passcode generation, and MFA approval through notifications.
There's currently no indication as to whether Microsoft intends to make this app available on other mobile platforms.