That is A pullout to attach A bathtub floor to the tarp. It might be the sun reflecting off A leaf in the yard but there is no flaws in it as far as I could see when I inspected it today.I was just looking at your pictures to get some idea of the work done. It caught my interest as I had done this myself on my pyramid tent. I sewed netting around the bottom to be able to lift it up and get some ventilation. Anyway, and I could be mistaken, but when I look at your first picture zoomed, it looks like there is a gap in the netting beneath the black patch on the tarp wall. Looks like there is a string running down from the patch, maybe it's a pullout there. That's not a gap though is it? The seam appears to flip over there possibly. Could be fine netting that I can't see maybe.