The button actually prevents unwanted noises — doing the exact opposite of the 4Runner’s Party Mode — and is part of the Smart’s optional anti-theft system.
The upside-down question mark represents a tow hook while the sound waves mimic motion detection. The button deactivates the Smart’s tow-away alarm and interior motion sensor. When these security system functions are armed normally, the alarm sounds when the vehicle is raised like when it’s on the hook of a tow truck (or possibly pranksters gone Smart tipping?) as well as if motion is detected inside.