Trelane's Lockbox is located in Stromgarde Keep of Arathi Highlands. [18, 67.9]
Contains [Trelane's Ember Agate] for the quest  Attack on the Tower.
In patch 3.0.2 this lockbox was replaced with a Rusted Prisoner's Footlocker, which is supposed to only be in Violet Hold, Dalaran. The new lockbox requires [Rusty Prison Key] to open and cannot be lockpicked.
A search for  Attack on the Tower on the official WoW forums reveals this as a recognized bug in Patch 3.02.