The S3 was entered as a custom model by someone at some point and it hasn't yet been approved by an admin. As we mention in the FAQ: "If you entered your vehicle by selecting 'Other' from the list of models, it might still need to be approved by an administrator for public browsing. Keep in mind that we don't approve all 'Other' models, so contact us if you have a question about a specific case."
I just approved it so it should be set. In the future you can just use the Contact Fuelly link at the bottom of any page to ask about why a specific model isn't listed the "Browse Vehicles" section.