Does the level sensor have 1 or 2 wires? Try unplugging / jumpering the wires to see if the warning goes away or changes to something else.

I'm not sure if MB still does this but on my 97 C280 the coolant level sensor circuit runs through the windshield washer level circuit. If the WW level is unplugged, the coolant level light will come on. This system works by reading resistance at each switch to determine what is low. It seems they did this to save an input point to the dash.