According to page 14 in my mannual for the 500 it is
"Brake pad wear indicator (yellow) Lights up during braking if the front wheel brake pads are worn down see page 53"
PAGE 53
"The brake pad wear indicator in the instrument cluster lights up when the steering lock is turned to driving postion "2" and must go out when the engine is idilong. If the indicator lights up during braking , This shoes that the front wheel brake pads are worn.
The brake system should be checked in a MERCEDES-BENZ service station as soon as possible"
This might have changed on the new cars to