All of my alts are in a casual guild that has a bunch of real life friends in it on Blackhand. With the new guild bank in 2.3, all of the old bank alt stuff got put into a four tab guild bank. The first tab was "Freebies" that anyone could access. Tabs two to four were all officer only access with the more valuable stuff.
Well it turns out last night a new initiate had full access to all of the tabs of the guild bank. That initiate stole 80%+ of the valaubles out of tabs two to four and then either deleted his toon or transferred to another server. So the bank logs list the initiate as "Unknown" for removing everything.
According to Blizzard, this is how they did it:
Known issue with Guild Bank withdrawal limits
"At this time, Guild Bank withdrawal limits for a given character are not reset when they join a new guild. Thus, whatever limits were applied to that character in their former guild will be carried over, and must be manually readjusted by the Guild Master in order to ensure the desired settings."
So the short term fix for this is for only the guildmaster to invite someone and manually setup their guild bank settings immediately after the invite.