When I upgraded to version 6 on my one controller it upgraded my controller 20 miles away? I couldn't figure out why my users in the other building couldn't use the wireless and when I got there it had version 6 as the primary. I did notice that when 6 was starting up it it asked me if I wanted to autoupgrade and proceeded to find my dhcp server and set up its own tftp server. I am assuming that is how it did it.
These controllers are not in any of the same mobility groups etc. I know the controllers talk to each other because once in a while one building's AP will go astray and use the controller in another building.