I'm glad you brought that up 543. The 'loss' of the phone has been troubling me too, so it's good to have a chance to re-explore that.
I can think of two possible scenarios for losing the phone, whilst it remains switched on, that involve a killer known to Allison. They are based on the triangulation reports of the phones location being within 150m of the Baden-Clay residence. Maps indicate this means the phone is on their property or an immediately neighbouring one, not down the road any great distance on a suggested walking route.
1) ABC is involved in a confrontation at home which leads her to fear for her safety. She runs out into the darkness with her phone to attempt a 000 call. She keeps as quiet as possible (no screaming) to avoid being found. The killer finds her easily by the light of the iPhone screen, snatches the phone and throws it away whilst muffling/strangling ABC.
2) ABC is involved in a confrontation at home, resulting in her death. The killer plants the phone nearby, leaving it switched on knowing it would be triangulated to the local area, to ADD credence to the 'Allison went for a walk' story.
Possible reasons for phone to remain missing
A hand & knee search of the area was carried out but was apparently fruitless. Did this include metal detector sweep of every inch? I don't know, perhaps unlikely - does anyone know? Regardless there are three possible reasons for it to remain missing:
1) phone was simply missed during search due to human error
2) search zone based on triangulation data was too narrow
3) phone was buried or planted in an inaccessible place
I'll share a failed 000 call story with you all in my next post - which could explain attempts at dialing 000 that never connect and are not recorded, even on the phone itself. This very situation happened to me a week ago.