Garry
it might be worth sending your findings to the admins at FlightRadar24. On all the occasions when AP-BKC is shown, there is also a valid Iberia Airbus on the same day. I checked out all the spurious flights on FlightRadar24 and all show a red or yellow question mark at the location, ie insufficient data. Quite why AP-BKC is substituted in the log for uncertain data seems strange. On the 20th May example, the spurious flight appears in the Bay of Biscay just as the Iberia Airbus lands at Heathrow - probably not a coincidence.
There are several sources of information as to just what is transmitted by Mode-S. These include (from memory) Mode-S code, flight number, flight level, position. As you say, a lot of interpolation from look up tables for registration and route has to take place using enthusiast-generated information. Every time Brazilian Boeing 767 PR-BRW appears on screen, I have to remind myself that it is really N637TW, 3 years after it changed registration! Then of course we have Voyager ZZ330 which was still flying around for several weeks with the Mode-S code of G-VYGA
But it still doesn't explain why AP-BKC appears in that data on FlightRadar24.
As for subtle location errors, FlightRadar24 merges data from many sources to produce an overall picture (as yet I don't contribute). Errors in PC clocks, latitude/longitude of receiver as entered by user, etc, all contribute to potential aircraft location errors. Recently on my system, which shows another source of shared data, the arrivals at Charles de Gaulle suddenly seemed to be located near Mablethorpe. Short term error but it indicates the type of underlying problem that can occur. Given the human input into all this, I'm amazed that FlightRadar24 works as well as it does.
And certainly delays over the internet contribute. I can see live data from my Mode-S receiver but if for some reason (eg a roof joist

stops reception, the shared data is shown. The jumping back and forth between live and shared data can be quite obvious.
Hope all that helps everyone.
Ian