I have on any number of occasions wandered across the San Ysidro-Tijuana border on foot where there is no kind of immigration control whatsoever departing the US or entering Mexico, hailed a cab to TIJ, and boarded flights to both South American countries via MEX or Asia via AM's nonstop TIJ-NRT flight. I've gone as far as Indonesia that way. Prior to WHTI being enforced, I reentered the US on foot using my California Drivers licence as ID. Even that they rarely scanned into the computer and just waved me past leaving no record whatsoever I ever set foot out of the US. Post WHTI enforcement they scan my passport or passport card but still have no record of when I left on foot and generally just presume I merely made a day trip barhopping or shopping in TJ like 99.99% of all the other American citizens returning on foot from Tijuana at that border crossing. If they specifically ask me I will honestly answer where I have been but the most questing I ever get is "what were you doing in Mexico" and I answer truthfully "going to visit friends" (I never see that they asked me WHERE was I going from Mexico, or where my friends were, only WHY I was in Mexico. I was in Mexico to GO to visit friends).