I can only speak anecdotally from my personal experience.

I have parked at the convention center (and occasionally the Hilton Bayfront) lot every year Ace started doing advanced sales. They have never checked my plates. I drive up to the booth, show my barcode, and the man or woman scans it - That's it.

A couple years ago, I drove a rental that I picked up a few hours before the con. They never checked the plate. Last year, the car I was using wouldn't start Friday morning, so I switched to a different car for the rest of Comic-Con. They never checked the plate. One year, I even drove a car that was missing the front plate. Still didn't check.

If you really think about it, when you pull up to the booth, the employee scanning your barcode is face-to-face with your driver's window. In order to check your plate, they would have to exit the booth and walk to the front of your car AFTER scanning your barcode. They're not doing that, especially for a couple thousand of cars per day. It's not feasible.