Another thing to check is the accessory bolts where the go into the heads etc. I had a mystery
oil leak on a car and even the dealer couldn't find it. I went investigating and found one of the bolt holes where an accessory bolted on. When it was drilled had gone all the way through the casting into the rocker box.
Oil was wicking out along the thread and running down the block. I fixed it with some thread sealant and a copper washer on the bolt.
Cheers