The empty bag caller is an item released with the Tools for Games update of 15 July2009. It can be bought from Faruq in Al Kharid. It can be used to send announcements to your friends chat that you either have no items in your bag, have no equipment being worn, or both. Attempting to use it while not meeting the requirements will return the message "The bag caller can't prove that you don't have equipment when you do." Attempting to use it again after recently using it will return the message "You cannot do that yet" in order to prevent spam. This item is useful for playing scavenger hunts.
There is no fool-proof way to cheat with this. If you drop your items and then use the empty bag caller, it will wait until your items disappear and then it will send the message. Mod Poppy has confirmed this.