fluid on friends driveway

I took the 328 out for a drive and stopped at friends place. When I left he called me and stated there was a puddle of oily yellow fluid in his driveway. I had to get to work so I didn't go see it then it rained hard and washed it away. There has been no fluid on my garage floor but his driveway was at an incline with the back parked on the down side. I have redline in the gearbox. The engine oil level was full and good color. The expansion tank for the coolant, which is green, was down an inch or so. Any idea what oily fluid is yellow.

 

I don't have any fluid on my garage floor and the car has been parked there after drives for the past year. I looked under and there was no trace of leak at the fill or drain plug. Competizione changed the gearbox fluid last summer and on the receipt it just states Redline. I would have thought if it was the gearbox there would be some on the garage floor after all this time. I ran it today and pulled on an incline with cardboard under it and let it run and no drips. The transmission shifted well after warmed up and no smell of oil on the exhaust. Has me stumped. The car goes into Competizione for engine oil change April 8. I called and talked to Richard and he thought it may be the coolant, but mine is green not yellow. They use the Mercedes coolant and it is yellow. He thought they changed the coolant when they replaced the water pump last summer.

Can you get the crush washer for the inspection/fill plug at NAPA and what number would that be or do I need to get it from Ricambi? I looked through the sticky but it wasn't clear since some posts stated the part number listed was not correct but they were referring to the drain plug.