Look, first of all, if we lived in a perfect world the Police would never get it wrong - even if such and idyllic scenario did exist I'm sure we'd still have scumbags in our midst.
Let me tell you that, with a missing person's enquiry, which the Tia Sharp murder began as, the Police are actually very restricted in what they are allowed to do. And the vast majority of 'mispers', as they are commonly known within Police circles, turn up safe and well.
Initial action would require details of the 'misper' to be obtained and circulated to other Forces, relevant people to be spoken to, i.e. family, a cursory search - if a child is the 'misper' - of his/her bedroom. It certainly does not require full blown searches with sniffer dogs etc etc.
In a genuine 'misper' case, family feelings are paramount, along with the (hopeful) safe return of the individual.
So many people are prone to talking with hindsight in the Tia Sharp tragedy and unfortunately have no idea how these enquiries actually take shape. Little is so far known of the specific details, therefore I'd be rather cautious about just spouting off willy nilly.