Based on some recent photos I saw, I'm guessing the same part of the border fence has been cut open that was cut in the spring when I was last there. It's about 2 miles from Lukeville.
Does the Border Patrol know where this cartel-controlled "gate" is? Of course. Why don't they weld it up and post a guard 24/7? Well, because then the cartels who control the U.S.-created border chokepoints would go 5 miles west...or 10 miles... or 20 miles and just cut a new gateway.
As it is, asylum seekers often walk the 2 miles and turn themselves in. If the gateway were 10 miles away, Border Patrol would have to spend all day driving out there to pick people up.
You cannot stop people with walls and fences. People will come so long as their situation at home is untenable. The border fence is just the last of many barriers they had to face to get here.
You cannot secure a border without a secure world. You cannot stop people with walls and fences.