Large companies have a hard time keeping secrets, especially when multiple partners and countless employees become involved. Bits and pieces are bound to leak out, both intentionally and accidentally, and sometimes a wealth of information can be discovered about unannounced plans. It appears that an app designed for Google's much anticipated MVNO has turned up in an unofficial Nexus 6 firmware image. If legitimate, the app not only confirms the service, but it may reveal some details about service plans, billing, and even a previously unheard-of name, "Project Fi."
Disclaimer: No matter the confidence level, there's always a chance product updates, features, and some or all details will be changed or cancelled altogether.