If you have full guild bank access and leave your guild and join a new guild your old guild bank permissions will remain (unless manually changed). People have been robbing guild banks and in the log it shows Unknown as the person who did the transaction.. Havent seen it here so heres my post.. and sources.
Also tested and confirmed.
http://us.blizzard.com/support/artic...8&pageNumber=1
Robbing the guild bank - WOW Insider