If they're traveling with partners, then it would be very difficult for one to complete the challenge and the other to not, wouldn't it...

[ That was also not really a question. Right now, they're both really just thinking out loud at each other. ]

Or perhaps those that disappear without reaching the challenge are simply people who've come across too much information. And whoever is behind all of this is just covering his tracks, keeping anyone else from finding out anything more.

[ ... That doesn't sound familiar at all. Totally not what happens in a certain someone's future... ]