“CBP notified the trespassers which areas would remain open”
Congress deliberately wrote some exceptions (gaps) into the appropriations they provided.
They purposely left 5 gaps unfunded, and required consultation with local (municipal) officials in 5 others (Salineno, Roma, Escobares, Rio Grande City, La Grulla).
But the net result is about 90% of the open mileage is getting built, and Border Patrol Officers and technology will converge on those much smaller gaps. If they can get through the administrative hurdle of local consultation requirement and get those small municipal areas built as well, only a few miles of gaps will remain.
Those purposely unfunded gaps are Santa Ana National Wildlife Refuge, La Lomita Historical Park, Bentsen-Rio Grande Valley State Park, within or east of the Vista del Mar Ranch tract of the Lower Rio Grande Valley National Wildlife Refuge, or the National Butterfly Center - something like a mile or two each.
Although the appropriation specifically prohibited using those specific dollars to build barrier in those locations, it did not specifically forbid ever building barrier there. Other money might be used, but it is probably just not worth the hassle right now, when getting the big gaps built is the priority.
Those small gaps will be like shooting fish in a barrel for the Border Patrol, compared to the over 100 miles of dense vegetation they have to police now.
“...like shooting fish in a barrel...” Figuratively speaking of course.