This test is not specific to Mercedes, it is called potential for draw.
This test will show you if there is anything drawing power while the car is not running. Make sure doors are closed and no lights are on.
Remove the negative lead of the battery
Place a volt meter between the negative terminal of battery and removed negative cable.
If there is something drawing power you will see a voltage reading.
To isolate begin pulling fuses until you determine where the
If there is no draw you can open a door to turn on dome lights to determine that the test is working.