Sometimes... They can do dual checks in/out at any time, they just usually don't. I got checked crossing at Lewiston "leaving" the USA by USA customs, and of course by Canadian customs upon "entering".
Thats what I am thinking... I have only been hounded by one side (US) and thats cus they like to hound and actually the last time I crossed, they had reason to mess with us and they didnt really.
Bookmarks