Originally Posted by
SEA1K4EVR
Yes you would because you'd have to go through US customs preclearance at YVR in order to board your onward flight back to the US. YVR is one of those airports where the arrivals level at the gate is separate from the departure level so to get from your arrivals level to the US departures level you'd be forced through US customs. I don't have experience at other Canadian airports but I'm guessing it's the same wherever there is a preclearance station.
I doubt UA would ever do this.
Yup
And a word to the wise-always travel domestically with your passport. I learned the hard way and missed out on a very lucrative bump and re-route via Canada because I did not have my passport with me (and as a result of that, I now always travel with it on domestic flights)