Can't fly out while you have a vehicle in Mexico?? True?

We crossed into Belize several years ago and told the Belize officials that were planning to leave vehicle in Belize and fly to Cista Rica. They told us that this was illegal and would have to leave rig at impound yard in Belize City. We did this and it worked well though we had to pay about $5 US per day. Good to hear about impound area at Cancun airport if we ever had to return under emergency conditions