Originally Posted by
tentseller
????
When you leave by land to US your passport is swiped by CBP. When you return by land CBSA swipes your passport.
I also heard that US DHS CBP is sharing information with CBSA now.
Again, I'm not claiming it works, but what you're suggesting does not necessarily invalidate what I said.
Various other posters above said ServiceCanada gets their info from the forms you complete upon return.