The US has no formal exit control. Itās handled by airline passenger manifests and a government database system that links individuals to passports, even if you have more than one, by a matching algorithm. I have checked in for flights out of the US with my Canadian passport and no one has asked to see my US passport. Coming in is a different matter. The airlines donāt care because Canadians are exempt from ESTA so they will board you. US CBP expects you to present a US passport at passport control to come back in, though. Other passports that require an ESTA or a visa can be problematic because US citizens are not eligible for those and airlines want proof of your ability to enter or they wonāt board you. Of course, the US could specifically intervene to stop you from leaving if they wanted, but in that case Mexican and Canadian land borders are poorly policed and easy to cross. The US and Canada share information on land border crossings and all travel by individuals with visas and permanent resident cards but supposedly not on citizen travel to/from airports that donāt involve both countries.