2004 Volkswagen Passat Oil Leak Under Valve Cover
Oil can leak internally, onto the spark plugs, without any external signs of leaking. This is a minor leak, as the tubes are not filled with pools of oil. The scant amount of oil is burning and baking into the tubes, though. This can eventually foul the spark plugs and / or ignition coils.
This will be remedied by removing the valve cover and replacing those seals.