Going back to the United States and forgetting your passport or border crossing card it is easy for Customs and Border Protection to check and see if they are US citizens, entering mainland Mexico is a different story RV's get a 10 year importation permit, all other vehicles get a 180 day importation permit, all persons traveling into Mainland Mexico have to get a 180 day visa, you get caught with out them, you get to go to a real nasty forced vacation place.
North Baja and South Baja and a certain portion of the State of Sonora have different requirements since they are a different fiscal territory, all border towns and up to 25 kilometers South you do not need any documents if you only stay for 72 hours, after that you are required to obtain a "permiso fronterizo" border permit but of course no one does that lives on the border towns that are not Mexican.
navegator