* A non-standard aftermarket addition: Someone may have installed a custom system or accessory with that labeling.
* Misidentification of another button: The button might be something else entirely, and "SGS" is a misreading or misunderstanding. Could you describe the button's location, appearance, or surrounding controls? This might help determine its actual function.
* A typo or misremembering: The button might have a different label altogether.
To figure out what it is, you'd need to provide more information about where it's located and what other controls are nearby. A picture would be very helpful.
