It is a tracking device. If you try to fill the mug at a resort that the mug was not purchased at it will deactivate the soda machine so you can't fill up and alert Disney security.
I seen this happen at the CBR, a guest had a mug from the AS Sports and was going to fill up at CBR but when he tried to fill it nothing happened because of the tracking device on the bottom but he didn't know that. Within about 3 minuets while the guy was still trying to figure out why the machine wouldn't despense his soda 2 security guys came and confiscated his mug and the other mugs from his family of four. His wife and kids were sent back to their rooms at the all star under house arrest for the rest of the day and he was taken in handcuffs to Chef Mickeys where he had to wash dishes untill the Resturant closed that night, afterwards he was taken to the MK where he had to clean every restroom in the park using only a Mickey Mouse
toothbrush, each restroom was inspected by Mary Poppins for her aproval before he could be finished, he had no memory of what happened after that but woke up the next morning at 9:30am proped up against his room at the AS Sports wearing a Peter Pan costume and bare feet. His family had all priority seating privileges taken away for the remainder of their stay and when it came time for check out they had to pay the hotel rate as if they had stayed in a tower suite at the GF. So remember, you an only fill up your mugs at the resort they were purchased at during the stay they were purchased.
