For those of you who have the Classic Controller, did you notice the triangle button on the back of the controller that releases the slots?
The article is old, but I assure you the button is still there. Here is an excerpt from the article:
There is a mysterious clip on the back of it which points to the ability to snap the classic controller in or on something. There was no evidence to say that the actual remote would go there. When questioned, the representative said, "It's a secret."