Remember there are 2 lights. one for passenger airbag on and one for passenger airbag off depending on occupants in passenger seat
http://www.mbusa.com/vcm/MB/DigitalAssets/pdfmb/ownersmanual/2015_SLK.pdf
page 41
PASSENGER AIR BAG OFF indicator lamp :
and PASSENGER AIR BAG ON indicator
lamp ; are part of the Occupant Classification
System (OCS).
The PASSENGER AIR BAG OFF indicator lamp
informs you about the status of the front passenger
front air bag.
If the PASSENGER AIR BAG OFF indicator
lamp:
Ris lit: the front-passenger front air bag is
deactivated. They will then not be deployed
in the event of an accident.
Rdoes not light up: the front-passenger
front air bag is enabled. If, in the event of an
accident, all deployment criteria are met,
the front-passenger front air bag is
deployed.