From your description, it appears to be the universal symbol for "brakes", albeit, drum brakes as the symbol is drawn to depict...so it is possibly, the brake wear indicator.
I don't know how early MB put in this feature, but there are wear sensors in the pads that as the pads wear, the sensors contact the disc surface, thus grounding them and completing the circuit that allows the indicator lamp to glow.
If I am correct on my guess, your brake pads are due for a replacement...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car