That's the 'Check Engine Light' which is triggered by absolutely anything the computer detects isn't right with the car. It can be triggered by something as small as the fuel filler cap not on properly, so if it stays off don't worry. If it persists you'll need to plug the car into a diagnostic reader which will identify exactly what the problem is. Mercedes charge just for a plug in, but many independant garages have readers and will do it for free. You can even get a diagnostic app for iphones now!!