With modern tech, yould could just drone any contraband over the border - and possible get a person over on a drone. Failing that a hang glider.
However "Soft Border Patrol" suggested there are plenty of country lanes that simply cross the border. Miles from anywhere.
When I was in Kenya, in the Masai Mara, we "popped into" Tanzania. By which I mean we circled a concrete obelisk. The guide said the border was only on paper.