Guild Bank Hacked
Odds are it wasn’t “hacked” so much as robbed by a former member of the guild. Have you looked to see if anyone has left your guild around the same time the items went missing? If so, that person may be the culprit. I have heard several stories about this, and it was usually done by someone (either being done as a troll or a hacked account) who joined the guild, waited until they could withdraw from the bank, cleared out anything of value, and then either deleted the account or just plain left the guild.