I don't have any experience with Smart cars, but over the years I've seen several MB's in my shop where the airbag(s) and/or ETR's deployed for no apparent reason. In every case, we found some form of underbody damage. I've seen subframe damage, floor-pan damage, and lower radiator support damage cause SRS deployment. Since the airbags/ETR's deploy almost instantly upon impact, the driver was never aware they hit anything. I'm not saying that an unintended deployment can't happen, I'm just saying that I've never seen it. I'm betting that the dealer will find this to be the reason in your case. Let us know what they find.