I've never had CBP check my passport when walking into Mexico. And Mexican officials are always asleep or absent when I pass through their side. No matter, all they require to enter Mexico is a US Birth Certificate (but they don't ask) lol. People leaving by private boat or plane are basically on a trust system; how the hell would they know if you're flying to Hawaii or not if headed to the Pacific, or whether you're going somewhere foreign or to Puerto Rico or the USVI if you're sailing into the Caribbean?
I'm pretty sure there's thousands a day that walk into Mexico everyday without a passport. Hell half the people in line returning to the US claim they "forgot" theirs.
Leaving is easy without a passport. And don't forgot if Sunny can get to the shores of Pakistan, he is a citizen and they must let him in, after which he can easily hide in the tribal areas. All he would need is a yacht somewhere on the multi-thousands of miles on the coastline and some friendly boats in int'l waters to refuel him.