I don't know if all Mercedes have it but mine have had a vent that screws into the top of the differential. This can get dirty and/or clogged up.
Some are hard to remove and some are not but it's a good idea to check it and clean it if possible. One MB technician told me he squirts some sort of cleaner on it if he can't remove it.
Mercedes (in the past) has only included a check for level of the differential lube in their service recommendations. On the other hand, BMW recommends replacing the lube at specified intervals which is what I do on both my Mercedes and BMW's..