I hiked the Long Trail in Vermont in 2003 and you walked up to the US-Canadian border in the woods and could stand with one foot on either side of the line. I believe there were planes patrolling the border. Around 1977, I paddled the St. Croix River, with forms the US-Canadian border in Maine, for several days. One night we camped on the Canadian side, in a mosquito-infested swamp, and no one was around who cared.