The Forgotten Lockbox is located in the Tavern in the Mists on the second floor. It only contains gold, 10g on average. It will also give experience points if the character is below level 90.
Does not appear to count toward Riches of Pandaria achievement.
"Post constructively or I'll press some different buttons next time." -- CM Daeleht (from the old WoW website, no source link)