When I took the whole thing apart, I found out that when I'd installed the little rubber elbow from the breather box to the block, I'd gotten one lip (bottom-left) of it folded under itself just a bit before clamping it on. In that spot, the rubber had failed and cracked and under some conditions and I was losing measurable oil through that crack.
Point being—don't discount anything.
In the end it's just got to be traced.