I have sat and watched (yes, I need to get a life) from my balcony at BLT in the mornings, and again in the evenings. I saw CMs in little golf carts riding around, checking out cars in the lot beneath me. I watched them come back throughout the day, and make note of some cars. They actually got out of the golf cart and looked at the car and wrote something down. Granted I only watched for about 15 mins midday. But later, just prior to MK closing, I watched them do that again...many stops were made.
Now, I can't say for sure what they were writing down. But, they did seem to be stopping at cars that did not have the white parking pass displayed. So, who knows.
I do know that when asked, I recommend people park in the further reaches of the parking lots if they aren't resort guests. But, especially at BW, non-guests seem to love taking those closer to the front spots...which means that resort guests, returning midday, have to park further away from the resort they are paying a hefty sum to stay at. Then, after IllumiNations is over, and Epcot is closed, you should see how many empty spots there are!!! I have sat and watched, drink in hand, as the line of cars exits the lot at 9:30-9:45ish.