Okay, so for right now I think I'm leaning toward the following:
She ends up outside the arena, (maybe she had to throw up and it was an instinct to run out the nearest door?) I think she was on something, based on LEs responses in this vein:
BBM, that's a *really* weird thing to say. I think it indicates that they're aware of some inebriation and don't want to share it, which is fine. It helps us to know that, however, because it means she may have made decisions that aren't as logical or safe as she typically would. It also provides the reason to need to instantly exit the building I mentioned above. Moving on...
So she's outside the arena, and waiting around, out of smokes, bored, unsure what to do. According to locals the RV area is a busy spot, you can hear and see people tailgating often, there's a lot of activity before the shows. If I were her, I could see wandering over to see if anything is still going on, see if anyone has a cigarette, hang out with some people while she waits, etc. That sounds plausible to me. This time, though, someone was there, and they had a different idea.
I think she was taken from the RV lot where her purse was found. I don't think the sighting on the bridge was her. It's possible that someone acted friendly, offered her a smoke, seemed nice for a while, snuck the battery out of her phone so that she wouldn't later be able to call for help, or so that when she tried to call someone her phone would be dead and he could offer her a ride. Or it's possible that the battery really isn't significant, like LE says, and this same scenario happened. She may have even dropped her purse on purpose as soon as she realized she was in trouble, to help people try to find her.
Sometimes the obvious answer is the correct one, and her disappearing from where her belongings were found seems like the most direct answer to me at this time.