Originally Posted by
jpmcdonough
USA has no departure controls. It is up to the airline to make sure you have the right docs for where you are going but otherwise you are free to leave. I have heard that this is sometimes a problem for people visiting the USA on a visa because they can't always prove that they left when they were supposed to, and when they come back they get questioned, but as a US citizen this is only hearsay.
The US does have departure controls. They just aren't done by CBP. The airline is meant to forward a list of outbound pax and passport/visa details to CBP when the flight departs. They also used to have to remove the departure card from the passport and send that to CBP for processing, but with the cards no longer used (at least for I-94W), it's harder for the pax to tell if the details have been sent on and thus they likely won't know if CBP thinks they are still in the US until they try to reenter.
If you saw that the card was not removed by the airline, you could sent it to CBP at an address listed on the back of the card.