Yes it would be good to hear from Marshall and any others who have theorised over the years.
Reading an old article here -
http://newsstore.fairfax.com.au/app...0&sp=adv&clsPage=1&docID=news970404_0078_0264 - it appears someone from ECU was spot on: "Meanwhile, Professor Don Thompson of Perth's Edith Cowan University has put forward a profile of an ordinary person "almost indistinguishable in the community", who gets his kicks from sex, the publicity and confounding the police."
Random thought re the Pipidinny Rd site - the suspect would have had to drive the length of the road to the beach, make sure nobody was there (and hence was no chance to disturb him on their way back) and then drive back to the dump site so that he need only monitor the road looking East. Hence the elevated location.
What's interesting though is that Wellard wouldn't have provided the same protection as it is open ended at Millar Rd (parallel with the rail track). I wonder if this was him becoming more sophisticated with each crime, or whether we should be looking for a Pipidinny Rd situation for the SS dump site?
The same article also contemplates an east dump site:
"Meanwhile, Dr Ananth Pullela, the forensic psychiatrist, said yesterday he believed he knew where police should be looking for the body of
Sarah Spiers.
The killer was deliberately spacing the bodies to avoid detection, Pullela told ABC Radio.
"We found one south of Perth, this time (Glennon) in the north; I'm suggesting we should carefully be looking at bushland east of Perth," he said. "
Although Wellard possibly now coincides with Madora Bay (i.e. convenience rather than structure), though the Freeway was still many years off reaching Mandurah, so wouldn't have been as convenient as it is now.