It’s not coming from inside the engine I can tell you that. It could be one of 2 things. Since that is an actual ‘tube’ that has a positive seal at the valve cover I don’t see too many leaks on these but on occasion I see the seals in the valve cover leak a little and over time can cause a small amount of oil to build up in the spark plug well, all you need do to fix this is replace the seals in the valve cover which can be tricky since they are ‘pressed in’ but it can be done and the parts are readily available.
The other possibility and I see this from time to time is that when someone filled the oil perhaps they spilled some on the valve cover near the spark plug tube and it went down into the well so if the tubes that have oil in them are near the oil cap this is also a possibility.