Evidence of combustion inside inlet cam cover

[Airzone]

Hi all... I got a puzzle for you

When changing my cam cover seals on the weekend (exhaust side was slightly leaking), I came across something interesting. When I took off the inlet cam cover and inspected it I noticed evidence of combustion on the inside of the cover at cylinder number 6. There wasn't any noticable evidence on the exhaust side. There were 2 dark 'cone' shapes indicating that this had come from cyl 6 from both valves. The rest of the cam cover was clean.

There wasn't any evidence around the cam or lifter - but a cover is in place over cyl 5 and 6 on both sides, so you can't really tell. There didn't seem to be any damage to the top of the head.

Also, when I scratched the combustion marks it came away easily, indicating that there wasn't actually much of it, and it wasn't burnt in... There was also a fine coat of oil over the top, around as much as the other cylinders.

The engine runs smoothly with no strange sounds (except for a ticking I believe to be from a lifter in the exhaust side of cyl 1) and the car performs as well as any other stock skyline. I haven't experienced any pinging and the engine is as stock as you get.

Also, there was evidence of the inlet cam cover been removed prior to me owning the car - they lost one of the screws and used a different one.

I'm guessing that the combustion marks came from either sudden combustion while the inlet for cyl 6 was still open (like detonation I guess) or a slow, gradual escape of combustion gases.

What I want to know is

1) What causes this (is my guess right?)

2) How can I tell if this is from previously repaired engine damage, or is happening now?

3) If something is indeed wrong, that's involved in fixing it?