Originally Posted by
seawolf
Was this prior to security screening? If so, then it is probably to control access to the CATSA Trusted Travellers security lanes. If you don't have the card you should be able to go thru general security lane and then access GE/NEXUS kiosk post security screening. That's how it works in YUL.
Once, yes, I believe it was while partially into the security screening line that I was asked to produce my GE card. And, for what it's worth, one or both of my boarding passes was also marked "TSA Pre", which of course is what agents State-side inspect to verify you're in the right line. For the security screening, by the way, in spite of having GE I (along with others in that lane) were requested to remove all packed liquids and place them in the bins as well as fully emptying our pockets. I don't know if this was a randomize selection for a group of us (or everyone that day?), but this does diverge from the generally policy described on the CATSA web-site.
The second time if I recall correctly where I was prompted for my GE card was after clearing security and entering the expedited US pre-clearance customs lane leading to the GE kiosks.