The car becomes more valuable in time if the mileage doesn't increase, as each month you get another 1042(?) miles taken off the odometer reading at turn in. So, letting the car sit for 1 year effectively drops it 2 mileage bins down on the table. On the A3, those bins are pretty steep, as in you more than pay for the flatbed tow to not roll it over the next 5k bound driving it to the turn-in.
It's been a while, so I think you were supposed to indicate mileage at turn in, and the date for the turn in. The date computes the months from 9/18/15, multiplies by 1042, subtracts it from the stated odometer reading, and looks up the miles adjustment.
Now, if you continued to drive the car, one of three things happens, you stay under the monthly miles, and stay relatively close to it, the value remains more or less flat from now until 12/2018. If you are over, you will start losing value, but only at the rate you go through the extra 5k bins. So if you are driving up to about 1300 miles a month, you won't slip more than 1 bin in 24 months. depending on where you start in the bin also matters if you are at a 5000 multiple + 1, you have the most "free" miles over 1042 a month, similarly, being at 5000 multiple -1, you are in the pay for the tow crowd. If you are under per month, it all depends on how much under, and how long you wait. If you are just over a 5k bound, keeping it a bit to fall back under is probably worth it. If you are at 4999, you probably aren't getting under unless you really aren't driving it, then you might as well park it, save the insurance and license fees, and send it back in 2 years anyway.