A few years ago we entered at Nogales and exited at Laredo. The guy where we turned in our vehicle permit indicated that was all we had to do - we did not turn in the FMM. Months later we crossed at La Palomas and they knew right away we did not turn in the previous years FMM. The guy told us the penalty was $5US per day - but he did not charge us so I think he was kidding. We now make the effort at Laredo to cross to the entry building and park and go in and return the FMM - requires driving in circles. We have never had our passport stamped on exit. I have no idea what the rules really are.
