I was replacing all 4 spark plugs on my 2001 Volvo S40, I noticed a darking-red residue at the bottom of the 2nd cylinder after removing the plug, the plug threads are clean no residue on it, my engine runs fine, what could it be? is it leaking or burnt?

 

Maybe some oil had gotten in there at one time then burned up.

 

Probably previously spilled oil like tech said.