Brad, it's a concept film. It's not supposed to be realistic, it wasn't a thriller as it ended up being marketed as. The "hole" was meant to symbolise being trapped within the teenage identity stereotypes. You have to look beyond the obvious and think laterally to appreciate it.
Incidentally no, noone would've thought to look in an abandoned bunker. Hence the word "abandoned". It would have been assumed that noone could get in. If you recall they were all impressed Martyn was able to get them in.
You also forget that she was psychologically profiled and they didn't believe she'd done anything wrong, so there was no reason to detain her. The official line was that the deaths weren't suspicious, so the only thing that was being investigated was the unlawful imprisonment.
It's not like real life justice is any different. Courtney Love was never investigated for Kurt Cobain's death, despite having motive and despite several holes in the pathology report (the amount of heroin in his bloodstream, the finger rather than thumbprint). OJ Simpson was found not guilty for Christ's sake!