No, for all
you know the fence was underneath - but I'm familiar with how the Briars were constructed, and there was no fence under them because there was no
under them. The Briars were a mass of individual tubes twisting out from the concrete floor below the waterline. You could see through them. A fence would be visible through them if it were hidden amongst the Briars, which it wasn't.
Here's a clip where you can see right through the Briars from multiple angles - no fence visible anywhere through:
Here's a gif of a TBA construction shot laid over a shot of the Briar Patch - look through, no fence:
View attachment 769551
Here are 2 TBA construction photos from February last year after they started removing the Briars - there's some scaffolding, but no fence:
View attachment 769549
View attachment 769548
It's fine not to know about this, but there's no need to discount the fact that other people do. I'm not being smug, I'm just telling you the truth. You're the one saying that since you don't know it must simply be unknowable.