When you have a window with a normal top bar (not a borderless one), the contact list persists as a window whether or not you are online -- it's just empty, obviously, if you are all offline. That's a general principle in Adium's implementation of the contact list, and I guess it basically just carries over to the borderless version and leaves this behind. So it *is* your contact list -- which is empty. I think you should be able to click on it and move it around and so forth, just like you can with your non-empty contact list....
I guess it's a minor nuisance. But heh, if I were the Adium guys, I wouldn't put it on the top of my priority list. 😛
If you want, go over to their board and submit a ticket about it.