I've wandered over to Tijuana from San Diego having completely forgot my wallet and/or ANY ID whatsoever. Returning, US immigration took it pretty well and simply played me a game of 40 questions for re-admittance to the US. For the most part they can apply common sense and determine if you belong there or not by simply talking to you for a few minutes. Of course there is exactly almost zero percent chance an airline will let you check-in an international flight without a passport so unless you got drunk and lost it in the lounge at the departure airport, it is pretty unlikely you would show up without a passport.