Try not to beat yourself up too much. I know you fervently wish you could have seen her or known, but you can't.
But, with what you said about the pipe at her apartment, along with the map, makes me wonder if it was someone who lived in her complex. Someone who had watched her, knew her patterns, knew she walked alone. Someone who maybe even gained her trust. Someone who lured her while she walked to the party and dumped her that close to home-which is also close to his home...
Yikes, it reminds me of The Lovely Bones.