Had to top up the expansion tank and noticed a coolant trail down the right side (looking from the engine bay forward). Seems to be starting at the block off plug for the sensor. This is an all metal radiator, have no idea of the vintage. I was thinking if I have any kind of leak, I would lose system pressure. But I had a distinctive hiss when I removed the expansion tank cap. This after the car had sat for several hours after about an hours drive at highway speeds. Should I be overly concerned? Or just put this on the list to replace the radiator soon? What about the block off plug? Anyone had one leak? What was the repair?