Such an odd and sad case. The location of her body, off her known route, is particularly curious.
Could she have tried to run from her assailant and been caught up with there?
Or was she moved against her will, either before or after sustaining the head injury? That might explain the missing jacket somewhat, as perps will often drag a person by their clothing, causing it to tear, fasteners to break and the garment to come off completely.
The deceased looks fit and youthful on the CCTV and I wonder if that may have been a factor in her attracting attention.
Total speculation: I can envision a scenario where a disturbed/sadistic young man spotted her walking past his house at an early hour, and decided to follow her with SA in mind. Only some time later, once he was directly upon his intended victim, does he realise that she was in fact older and perhaps did not meet his intended ideal profile for a victim (we know that many sex offenders have a specific fantasy or ‘type’ they prefer to victimise, with youth often being a factor in this).
As is typical, the offender lashes out in rage at being ‘thwarted’ or otherwise disappointed in his intent, which could explain the head and face injuries, robbery and lack of reported SA. It could also explains him not ‘finishing the job’ and still leaving her alive. The offender then flees the scene, discarding the jacket en route, and passing the phone on to friends/family to get it out of his possession once he realised his panicked mistake.
Phone is tracked to the person currently in possession, who says they got it from X. X is found to live on the walking route, have a relevant criminal history and/or be seen on CCTV also walking in the early hours some way behind the victim.
Enough to arrest, but not to charge. JMO and pure speculation based on what we have so far.