As an Audi S4 owner, but a normal person and not an Audi driver, I'd think that the two dodgy guys with that Audi S3 had just got their hands on it and probably required your number plates for it prior to "using" it.
Other than fitting Autowatch Ghost, which I believe has some other enhanced features to allow you to part with your running car and still be able to get it to stop a few 100 meters later - but who has the coolness etc needed to trigger the "disable in a few 100 meters" feature just as they are being car jacked - I've never worked that one out , though maybe you would get it right after the 10th car jacking in a week!
I tend to use DiskLok, it is a pain in the neck to use, but if it helps me hold onto my car, I can live with that, I can see that your error was to unlock the car before you were beside it, not a good plan, especially as you then spotted some nasties blocking your car in, I'm sure that your default action when someone blocking your car in - especially nasty looking ones in an Audi S3, would be to walk on, better to live another day than prove that you are no match for hardened nasties.