With the several patents Apple already has regarding an embedded Touch ID sensor in the display, and today Apple has been granted another one. Filed back in 2015, the patent describes a fingerprint sensor that could seamlessly blend into the user experience.
Where a fingerprint sensor is integrated into an electronic device or host device, for example, as noted above, it may be desirable to more quickly perform authentication, particularly while performing another task or an application on the electronic device. In other words, in some instances it may be undesirable to have a user perform an authentication in a separate authentication step, for example switching between tasks to perform the authentication.
In other words, if an app such as the App Stores asks for authentication, but the user needs to switch away to another app, you would not see an authentication popup because the device has already read your fingerprint.
This would totally make sense as in cases, even with the existing iPhones, you’d already have your finger on the Home button so there would be no need to authentication. However, the Touch ID sensor on the existing phones aren’t reading your fingerprint unless it’s under a certain scenario (such as unlocking your iPhone, or authenticating with an app). But, an “always on” sensor could cut down on some time and could seamlessly blend into the user experience.