I am a complete unbeliever in the concept of a "shadow". It is true that as a WP or P1 you can select seats that are not available to most in pre-selection. Thus, the other flyers who select seats are likely to be initially WP etc and wont choose to be seated beside another pax by choice - hence the appearance of many empty seats between pax in these prime areas. When the plane is not full, these empty seats remain and thus you have your glorious shadow
But they are, in my opinion, just a myth.
I have never ever seen, EVER, any Qantas reference to such a concept. We on AFF love to beleive that they are there, and we even get peeved when said shadow does not "stick". I have seen that on many flights I can select a seat that is otherwise blocked (as seen on expert flyer or similar). But these are usually because I can get whatever seat, including bassinet positions. So if you can jag one of these you may see on certain sites that the seat beside you is blocked. But I am willing to bet my left test__cle that these are things that are not about shadows
Half the time the shadow gets occupied by a late comer with zero status........ A product of different seating parameters as a flight gets near to departure.