According to US law (and I am assuming Syrian law) that is true...assuming the government is stable and the country doesn't have a devastating stalemated civil

According to US law (and I am assuming Syrian law) that is true...assuming the government is stable and the country doesn't have a devastating stalemated civil war. Getting into Syria with a legal passport is difficult enough, and Syrian border officials would have good reason to suspect that he was a foreigner. He would likely be thrown on a plane to Cairo and told to figure it out at the Syrian embassy. Although now that I write it out maybe this wouldn't have been a terrible option (albeit Kafkaesque and prone to several bureaucratic failures - and it doesn't solve the problem of him "abandoning his post" and breaking Egyptian law). Even so, US authorities do not just take "sorry fellow USicans, I lost my passport" at face value and will likely detain you until you are able to prove you are a citizen, and put you through quite a bit of extra screening and interrogation. Realistically it would be less of an issue for a white person with a bland American accent, since Border Patrol would probably let you call a lawyer or family member to bring a birth certificate. But I am quite sure an El Salvadoran-American who forgot his passport would not be able to enter the US from Mexico without incredible legal difficulty - again to the point where it would be faster and considerably less traumatizing to just go to a consulate and wait a few weeks.