You may not leave your car behind in Mexico with only an FMT tourist permit. That is one of the conditions of your 'importada temporal', which is only valid as long as your immigration status is legal. Once you leave Mexico, your FMT becomes void and is actually collected if you fly out, making the car in the country illegally. If stolen, used in a crime, injured property or a person, you would remain liable and the vehicle subject to permanent confiscation.
The Mexican computer system has become much more capable and, this year, all of the agents have been replaced with university educated, stricter border employees. What would you say, if asked, "Where is your car?" the next time you tried to enter Mexico?
To leave the country without your vehicle, you must have an FM2 or FM3 visa. Then, you can come and go at will.
If you were caught, as in Spark's scenario, you would be subject to heavy fines and possible confiscation.