suspension

You may have an Active Body Control compressor leak or just a leak in the lines that feed the air through your suspension. If it is your front side then there may be an easy fix for that and the part is called a front strut air line leak patch. There is a gold piece on top of your front strut and you can test if there is a leak by putting water in a cup and put it over the gold piece with the line going through it. The line is what carries the air to each of your struts to keep your car level. If it is bad when it is off, then this could be drawing power from your battery as the compressor may be trying to level out your car when it is off and it can't until you turn your car on and add more power to your battery and hence leveling your car. All Mercedes have an automatic leveling system that levels your car out when you add something to your trunk and also when you take the items out of your trunk, etc. Check to see if you have air bubbles from the top of your strut by putting some soapy water in a cup or something that you can use to see if there are bubbles coming from the gold piece with the line going through it over your front strut.

happens all the time on the S55's, replacement valve is bad, parts and labor comes to roughly $2400 for one valve generally on the passengers side is where you find the problem, a virtual guaranteed problem after 80k miles