This light keeps coming intermittently as you mention, but I believe more frequently, on my 300E. Took it to the dealer who said they fixed it, and by the time I get home, it is on again. Took it back, only to develoope the same scenario.
Took it to my mechanic here in town, and he said it was the SRS bushings in the steering wheel not making continuous contact. A cheap repair that took very little time to troubleshoot and repair.
One more reason to not have a dealership ever touch one of my cars. They are also not owning up to the repair bill...sigh!
Maybe this will shed some light,