Originally Posted by
ESpen36
So the government would not care if you left by air, then returned by land or sea without a passport scan, and then tried to board another international flight to leave again? Or would the system be flagged because there was no record of you returning before departing again? (assuming a US Citizen/LPR)
I don't think there is any legal basis for "caring" how you entered the country especially if a border scan is not mandatory. Even for many countries that have exit control (not exactly but even if applied in the U.S.), having an arrival record in order to depart would logically only apply to foreigners.
The U.S. Government requires airlines to collect immigration stubs from foreigners flying out of the country and collect information of citizens boarding an international flight. These data are not part of APIS and most likely are processed at a slower pace--at least for the paper stubs.