Curious. That's not how things were signposted/worked last week.

UA issued our SEA-YVR, YVR-LHR boarding passes. At YVR, signs directed us into the immigration hall proper where there was a small bank of desks on the far wall specifically for dealing with transit passengers*.

We were given large pink laminated cards (TRANSFER or something like that), and then ushered to an elevator at the back of the hall, which popped us out just by Starbucks in the international departures hall. No additional security.


*these were actually closed because it was so early in the morning, but the main immigration desk performed the
same function and sent us back to the lift with pink cards

EDIT: I'm also fairly sure it's not as simple as it being in the same terminal. US flights are separate in Canada and DHS does immigration pre-clearance at Canadian airports (they arrive at the domestic terminals of US airports). Certainly our arriving and departing flights were in completely different sections of the international terminal.