If the body was floating, and the person was using a flashlight or waited for their eyes to adjust to the darkness of the culvert, she would have been seen. But also someone said she was found on the NW side of the culvert (or maybe they meant NW side of intersection?) -- it is possible that when LE/SAR searched she was fully inside it and over time the current moved her out. Without knowing the exact location of the body, difficult to know how visible she was at any given time. As I said, that's why I always carry a powerful flashlight during day searches.
There is a good question in all of this though - how long was she wandering around? Did she go directly to the creek/culvert? Or not?
There must be two culverts at the intersection: one running East-West, and another running North-South. She was found at the NW section of the culverts.