Luck of the draw, as to how deep they will investigate.
Make no mistake that they do look at things fairly closely. From the information that you submit, they can do a pretty good profile on you via your spending habits. Changes in these habits can easily be spotted, or the habit itself can give information, that may cause them to investigate further.
Exactly what they check out every time I do not know for sure, DVLA is easy access for them, so trying to hide a car is not a good idea!