Between Tijuana and Tecate there are still about seven miles of wide-open border. This post shows how it looks and issues are discussed that may explain why it has been left open. Some of it is rocky, mountain ridges, but still there are a few miles, where a person can walk across on relatively flat ground with no barrier at all to stop him.
Though it still needs some work, the state of the Border Fence is really pretty good along the rest of the 160-mile California border, tremendously better than it was in 2018, before the new construction started.
However, Marron Valley is just one place that was skipped for some reason. Maybe it is for a good reason? It is probably because this section presents a more difficult engineering problem than most other sections. So, it was not rolled out like other sections of fence on the border with a more standard, less expensive design.
You might compare this section with the small section that runs into the Pacific Ocean near San Diego. That section has not been replaced either. It would take a special design and a crew with special skills to construct a bollard fence that extends into the surf. So, that part has apparently also been left for a future project.
This map shows the layout of the area. It is about 11.5 miles from Tijuana (or Otay Mesa on the US side) to Tecate. There is about four miles of pre-existing fence, built around 2008-2010 on Otay Mountain. 1.5 miles of new double fence is being constructed on the west side to better secure the Otay Mountain area. However, there are about seven miles of border that has no barrier whatsoever. You can click on these images to obtain a larger view.
This map shows a closer view of the current gap and is at an angle compared to the previous map to better display the terrain. It shows where the fence stops on the slopes of Otay Mountain. The area of completely open border extends from there about seven miles across Marron Valley and across the slopes of Tecate Peak to the outskirts of Tecate.
This is a close up panoramic image of one part of Marron Valley at the end of Marron Valley Road in . The border would be about 100 yeards on the other side of the river in this area. You can see that there is no barrier on the border at all and a dirt road that crosses the border.

Here is a broader view of the same part of Marron Valley above.

I was told I could see into Maroon Valley from the top of Tecate Peak. The road can be pretty rough, but you can drive all the way to the top with an all-wheel-drive vehicle if the road has been graded recently. The distance is about four miles to the top.
This is an overlook of Marron Valley on a hazy day. You can click on this photo and expand it for a better view. You just make out the Tijuana River bed as a meandering dark line of trees/brush at the bottom of the valley. ( I may go back on a more clear day for better photos.)

Next is an Image of the terrain to the north of the border and Marron Valley. If you look to the very left side, you can see the Tijuana River as a dark splotch at the bottom of the valley.
Certainly there are many paths illegal crossers can take to hike across the border here to a pickup location on Highway 94. You can click on this panoramic image and obtain a larger, high-resolution view.
I did not approach the border from the Mexican side, either, as I sometimes do, because only dirt roads lead down to the border in that isolated area and there are serious drug operations taking place there. A young Mexican citizen, working for the US Embassy as an agent for the U.S. Department of Agriculture was killed in the area just across the border while collecting insects in October, 2020. The Narcos apparently thought he was spying on them. His body was riddled with nine bullets. So, I decided not to try to approach the border on the Mexican side for a better view!
The image below shows the fence as it ends on the slope of Tecate Peak just west of the town of Tecate. The town of Tecate has a new 30-foot bollard fence, but this smaller section going up the slopes of Tecate Peak is an older, 15-foot fence, probably built around 2008-2010. From where this fence ends, there is no border barrier for about seven miles until the fences starts up again on the east side of Otay Mountain.

Finally, here is a photo of Tecate and the border wall from Tecate Peak. The dark line dividing the U.S. and Mexico is a 30-foot bollard fence. I it actually about 26 feet high, but construted of 30-foot bollards, with a four-foot high anti-climb plate on top. The actual height depends on how deep the bollards are set in the ground.

I have not talked to the Public Affairs Officer for the San Diego Sector about why there is no fence, at all, in this area. I speculate that the reason they have not built a new fence here is probably due to the expense of building in this rugged, isolated area and also that it would be a significant engineering challenge to build a fence here. The likely reason is that the Tijuana river meanders back and forth across the border and also the run-off from mountain slopes drain into the Tijuana River. The river is probably dry or nearly dry most of the time, but when it rains there will be huge flash floods that could wash a fence away, even when it is set in concrete several feet deep. They don’t build in river beds. They could build a little north of the river, but there may be a land ownership issue with that.
It is cheaper and easier to build in other places and so this place was probably at the bottom of the list. They were trying to build as many miles of fence as possible with the funding that they had before Trump might have to leave office. Maybe they will be able to construct a barrier here in future years?