I posted one of our experiences in another thread some time ago. We got our FMM at one crossing and when we left at another crossing there was some confusion lost in translation with the guy where the car decal was scraped off. We asked and he indicated that was all we had to do. The next time we crossed they knew right away when they swiped our passport that we did not turn it in the last time. The guy said the fine was $5 a day. We told him our story and that the other guy told us we did not need to turn it in. He did not fine us. I am not sure if maybe it was his sense of humor or if there could actually/supposedly be any kind of find. Now we always turn it in. Point being - our experience has been that they do know if you turn it in or not. Maybe not at all crossings. But we have crossed at quite a few different places in either direction and they do seem to know - the computers are all linked.