I was thinking: Was she on drugs...
She was hooking up with someone else and this is her hare brained plan to not have her bf find out.
Re drugs:
According to the article her mother said she had a history of narcotic use and mental illness, so I’d say the chances were good she was stoned or having some sort of mental episode.
But three weeks?? In the sewers? What was she eating? What water was she drinking? Yikes.. maybe we don’t want to know...