How are they doing this, that is my point...
With online check in - you never hit the front desk. Who is asking who if they have parked a car, and when how is it asked.
And only 3-4% of guests staying at a DVC resort paid cash? I'm not a gambling man, but I'd wager this is not fact.
I've been trying to find out about that percentage. My figure was based on the percentage that Disney actually own at a sold out resort, which is 2-3% by law.
In addition to that, when a member trades their points for a stay elsewhere, like Adventures by Disney, cruises or non-DVC Disney resorts, the points are transferred to Disney to use. Those are the points available that Disney can use to take cash reservations. But those options are a notoriously bad use of points, so many owners will rent out points and pay cash instead.
Also, any villa not booked by 60 days out can be transferred to CRO for cash bookings.
Since DVC operates at around 95% occupant you year round, I would think the added percentage of rooms from unused points wouldn't be more than maybe 4%. Meaning the total might be nearer 7%, but certainly well under 10%.
The vast majority of guests will be either members or people who rented points to stay there. Not cash reservations.
As to how, with that small number it shouldn't be hard. Exactly how remains to be seen. But since they have contact details, it could be simply a text or email, asking if you have a car - if you haven't checked in in person or visited the front desk. At some point, someone may post somewhere about it. But other than UK guests who stay at OKW or SSR, who are exempt from the parking fees anyway, I don't see a lot of people who pay cash to stay at DVC resorts posting on forums.