Players are reporting that their guild banks are being robbed by new guild recruits. While this is a known issue for Blizzard, many players are still unaware of the bug.
Currently, withdrawal limits are not reset when a character joins a new guild. Whatever a character's old guild status was carries over. In this way, officers are leaving guilds, joining new ones, and gaining officer access to the new guild's bank tabs. After withdrawing most of the loot, the character transfers servers or mails off the loot and deletes his toon. The guild logs report "Unknown" as the culprit.
In theory, any character could start their own guild, or get invited as an officer to a new guild (this is common when children start guilds), just to exploit this problem.
The only fix is for the guild master to manually reset the new recruit's guild bank permissions immediately upon invite. While there will always be potential for abuse when pooling resources, at bare minimum, we should be able to protect our investments from outside forces.