Can you maybe describe a bit more as to what you are smelling? What does it smell like, i.e. exhaust, fuel, oil, coolant, etc. Is your air conditioner/heat on when you smell it? Check your cabin air filter to see if it's dirty. I've seen individuals report an issue like this and changed out their normal filter for a charcoal one, which solved the problem.
2008 Mini Cooper S Hatchback - Filters & Belts - Page 1