A video camera system (or in some cases, an actual human) checks the images snapped of license plates on each car that doesn’t pay, then checks to see if the car is registered to an I-Pass account. If so, the account is dinged for the lower toll, just as if the transponder had been there all along. It shows up on your statement as a v-toll — a friendly reminder to make sure your transponder is working. But there’s no penalty.