Look for people with a Rolls Royce registered who also claim to earn little money? Then what? Perhaps it was a gift from family, perhaps it was purchased with savings.
The point is, an automated database query is cheap and gives a shorter list of people to investigate compared to everyone in the tax durisdiction.
If you have a car whose purchase price is $40000 and you withdraw $30000-35000 from savings in the months before I think the database query could reasonably file you under the low priority investigation list.
If you have a car whose purchase price is $40000 and no transactions which match up with it, then you get filed under the medium priority investigation list.
And if you have multiple houses and no income or savings to account for the purchase costs then you get filed under high priority and a human taxman will start an investigation.
You use the cheapest method available to create ever shorter lists of people with anomalies to pass to the next, slightly more expensive filter.