I can see how "we're missing somebody" would come up in the conversation innocently.
So, imagine this: you're out with your friends and you've been tentatively planning to meet up with another group (DS's group). Your friend Morgan disappears -- gets locked out of the concert hall. Morgan tells you, "I'll try to find a ride with somebody." You're like, "OK, let us know when you do!"
You get into the music, kinda forget about Morgan for a little bit -- you assume she's making calls. After some awesome song ends, you realize you still haven't heard from her about her plans. You look at your friends and go, "That's weird. Someone try to call her."
Her phone is off. She's not answering. And she never let you know if she'd found a ride or not! How annoying. You try to call her again. She's still not answering/her phone is going to voicemail.
"She's probably fine," someone says. "You know her phone goes dead a lot/the battery is always falling out/she turned it off to conserve power. I bet she got a ride home with X."
"Yeah, she would have called if she couldn't find a ride," someone else adds.
Still, it's a little weird. But hte concert is still going so you let it go for now. Meanwhile, this other group you were supposed to meet up with starts calling and saying, "So what's up, are we meeting up afterward?" And because this weirdness with Morgan is still kinda on your mind, maybe even pissing you off a little -- how annoying that she got shut out of the show, when you totally wanted to see it with her, and now has turned off her phone! -- you say, "I don't know, one of our friends went missing, let me see if people are still up for an afterparty."
Well, then you end up bailing on the afterparty. Maybe, yeah, you're still a wee bit worried about Morgan on the drive home, but hey, she knows the area, she would have called if she couldn't find a ride. And maybe once you get home, you and your friends have a few drinks and b*tch/wonder about it when you still can't get through to her phone. But then the alcohol and fatigue kick in and you say, 'Okay, we'll check on her first thing in the morning. Night, guys.' And everybody passes out and, like most twenty year olds, don't wake up the next day until noon -- when it's Morgan dad calling, saying, "Where is my daughter?"
In short, I present this hypothetical scenario because I think there's a totally innocent explanation here. And I'm a little uncomfortable, to be honest, with the surfacing tendency to blame the friends for what happened to Morgan. If a perp took her, the fault and sin lie solely with the PERP. In retrospect, I'm sure M's friends wish they had done things differently -- like, called the police the MOMENT Morgan didn't pick up her phone. (But really, would the police have done anything right away if they'd heard that a 20 year old girl got separated from her friends at a rock concert and kicked out? I suspect they *still* would have waited around 24 hours, thinking that Morgan was about to show up on the arm of a new rocker boyfriend.) And I'm sure these thoughts are haunting them. But if something happened to Morgan outside the arena while her friends were inside it -- it was in no way, shape or form their fault.