The seat belt
fastened/unfastened connector is under each seat. It is a blue plastic 2-wire connector. If it is disconnected or loose, the SRS indicator comes on. Worth checking BEFORE a trip to the dealer. It's tough to reach unless you have small hands. Seat can be unbolted, but requires a star wrench, available at most hardware stores.
My passenger seat belt was defective when I bought the car. No light, but it would not allow movement - just kept cinching tighter and tighter. Then the passenger would unbuckle - and BEEP BEEP until they fastened it again. Very common problem on many cars, not just smarties. Universal smart center in LA replaced the belt under warranty. Pre-ordered it - took a month for it to arrive. Then took the car in, took an hour to replace.
evilution.uk has a mod to silence the alarm beep, but unfortunately on 451's it also makes the SRS light go on(false alarm).
Evilution - Smart Car Encyclopaedia
Good pics of the area though.
Most of the time the light comes on it's the belt - not the airbag. But check those connectors.