Help for Spark Plugs Issue

Today is my first time to check the spark plugs conditions since I bought the Prius on 2017, and found the threads of the fourth plug looked a bit wet, and there was some oily brown stuff on the threads, but the plug tip was dry without any black oil, I inserted a piece of tissue into the plug tube and didn't see any oil there, no oil on the ignition coil at all, and all other 3 spark plugs were fine. I have no clue how the oil only got onto the plug threads, was it an early sign of oil leak from the plugs "O" ring seal? my car was serviced at the local Toyota dealer, the engine oil was overfilled at the last service, don't know if it's the reason caused the issue, has anyone had the similar issue before? I hope there is no big problem behind.

 

Many thanks, the car has done ~65,000 miles so far, The EGR hasn't been cleaned yet, but I checked the EGR pipe and it looked ok, didn't see much carbon buildup inside the pipe and EGR valve passage, I cleaned both of them, and expect the EGR itself and the cooler should be ok and aren't severely clogged.

 

But I can't image how the oil was spilled into the plugs, as all spark plugs are covered by the engine cover. you are right, the dealer tech did mess up my car at the last service, oil was overfilled, the oil filter was over-tightened and the sump plug was under-tightened (slow oil leak).