2010 Toyota Prius Owners Manual - Page 272
2010 Toyota Prius Manual
Page 272 highlights
2-4. Using other driving systems When the Enhanced VSC/TRAC is operating If the vehicle is in danger of slipping or if the front wheels spin, the slip indicator light flashes to indicate that the Enhanced VSC/ TRAC have been engaged. A buzzer (intermittent) sounds to indicate that Enhanced VSC is operating. ITO24P109