The Juggernaut Alpha has always been a mystery device, but with the help of a developer monWindowsphone has managed to track it down to a building on the Microsoft Campus.
The anonymous developer revealed the device showed up on their logs with the following details:
The latitude and longitude places the phone slap bang on the Microsoft site, in Studio H, which just happens to be where the Windows Phone Design team are located (and who also has an awesome cafeteria).
The phone runs Redmond_83012_123215 firmware, suggestive of a Microsoft-branded device, and from previous leaks comes both in a CDMA and GSM version.
Microsoft has suggested that they are becoming both a software and device company, making an own-braded device nearly inevitable. We may just be seeing this monster stirring in its lair at the moment.