A few people have made this comment. However, I think it's complex because she has vanished off the streets of London and no one knows or is saying any reason why that might have happened, and they haven't found any evidence as to what has happened to her.
Hundreds of CCTV to talk to, many potential witnesses to track, acquaintances to interview, no evidence of a body, no indication that she has gone voluntary missing.
As someone said upthread, Missing Persons cases are usually solved within 24-48 hours. The team assigned at the beginning - from the 'my friend has gone missing' phonecall - are allocated to a Missing Persons enquiry, they deploy Search and Rescue etc etc. It is clearly more complex than your standard missing persons case.
ETA: what I mean is, I wouldn't infer from this that Sarah had a 'complicated' life, or that there are hidden complexities. Having said that, of course we all have complicated lives, and I'm sure there are many things they aren't telling us!