There are two tabs that further secure the mouse buttons to the frame. They are located on either side of the mouse cord retracting mechanism.

The tighter confines of the mouse interior will be easier to navigate with metal prying tools. While plastic tools are preferred for most plastic devices, they are too large to access the left tab in the picture.

To dislodge the tabs, use a prying tool while pressing down on the back of the mouse. When prying, push the tab towards the outside of the mouse.