The tape can be found at Camp Venture, in the command center.
Overseer's log. Camp Venture.
Just who was this Brotherhood of Steel? Survivalists? Former Army personel? They took their training and their call signs seriously, that's for certain. Organized, efficient, access to hardware. You'd think they'd be ruling West Virginia by now, but they're gone.
If they're following military logic, then they'll have a fortified headquarters somewhere. And a leader. Someone who knew what they were planning.