It was just a thought...I seem to have read quite a few reports of owners having faulty coil packs eventually giving an engine code fault. It occurred that if the integrated coil/anti-knock unit was malfunctioning then the combustion loads in that one cylinder could be higher.
There is this thread:
where plugs apparently hadn't been changed for 47K miles - is that normal?
The owner also ended up having 2 faulty coils replaced.