It's my understanding that the reason for replacing the airbags when they reach a certain age is because they may FAIL to deploy when needed.

At my dealership I get a handful of "accidental" air bag deployments every year. I've ALWAYS found underbody damage to be the cause. In each case the owner had no idea they hit anything, most likely because the airbag deploys just milliseconds after striking an object. I'm not saying it can't happen, but I've NEVER seen an MB airbag deploy for no reason. I'm guessing your dealer has had similar experience. They'll give the car a good going-over, and it won't surprise me if they find the cause relatively quickly. Let us know what they do.