just completed another search run at 600ft along the beaches south of Perth past Bunbiry, Busselton, Dunsborough, Cape Naturalist and down almost to the Leeuwin lighthouse.
there is absolutely no hint of wreckage or flotsam of any kind washing up.
the beaches are amazing. there are people everywhere on the beaches.
between perth and bunbury there would be a vehicle camped/parked about every 400 yards along the beach.
between bunbury and cape naturalist there are people walking and swimming on every beach.
south of cape naturalist there is about 1km of deserted beach. south of there there are people and surfers everywhere.
so the likelihood of anything washing up on the beaches and remaining there unnoticed is I think absolutely nil. I think if anything washed up it would be noticed within half a day.