It is probably a leak from the top center head bolt. Inside the rocker chamber is a big bolt head or a big allen head plug which covers the top center head bolt. This plug had a crush washer on the old bikes and an o-ring on the newer ones. It is a VERY common source for head leaks on a Guzzi.
Patrick: Thanks for that info; I've got a similar leak on my Ambo, and was suspecting that the little cylinder head bolt seals got tweaked when I put it back together. I like your idea better...