I actually go here often. It is a mile each way to walk across it and if you stop to take pictures/look at the view you can spend quite a long time on it.
I'm assuming they didn't get footage of her coming back? If you cross the entire bridge there isn't a lot on the Missouri side, a riverfront park to the east that typically is pretty empty and a highway to the west. It's possible that you could walk across the bridge and keep going on the riverfront trail all the way to downtown St. Louis, but I would think very few people would do that - it's a bit sketchy. There are also some neighborhoods in the area also.
As for the bridge, this is not a place you just accidently end up at (on the Illinois side) - you have to drive down a long road, then there is a one-way bridge you have to cross and then more road before you even get to the Chain of Rocks bridge. When I have been there recently, typically I pass around 6 to 10 people walking or riding bikes on the bridge but you are alone for large stretches usually. Almost every time I've gone, there are people below the bridge fishing who would have a good view of the entirety of it. They would probably not here anything thought because the water is pretty loud there.
As for SLU - this wouldn't be my first way to SLU but it would be possible to take a round-about drive and stop at Chain of Rocks first - it would be a very long drive though.
I really hope she is found safe and soon.
I may be off but I think madison county is at least an hour from Florissant and not in the direction of SLU
