I would check for a large drug shipment in that area. The Mexican army had to keep the border agent busy to provide cover.
It may be time for another mexican American War only this time we’ll hold onto the land we take and politely ask the residents of the expanded border to pack it on south (No more Senator Salazars). The Hidalgo Guadalope treaty drove us back to the current boundaries although we had driven them to mexico City. Our chief negotiater gave away the Baja Peninsula although the president explicitly forbade him to do that.
The real answer here....is that a fence really existed...you couldn’t accidentally or purposely cross the border. Yet, you won’t see a single senator push for a full-up fence all the way across.