FR24 is following the FAA BARR (Blocked Aircraft Registration Request) list
http://www.nbaa.org/ops/security/barr/
Some planes will not even appear in FR24 or in the Archives. The BARR list is specific...a/c on the list should not appear on any public websites.
The process is easy to be added to the BARR list and was recently changed (Aug) so there are more and more Aircraft opting in.
Plane Plotter is a private/commercial website, so they don't use the BARR list, however FlightAware/PlaneFinder do follow the BARR list.
The aircraft in question yesterday was a US registered G450
If you have the FR24 App, searching for BLOCKED will normally produce 1 result all the time.
On a side note, it would be nice to have a car icon for the ModeS cars in SYD