A therapy session in which I talked about what I found to my therapist helped me to realize that I am so good at operating in black/white terms that I always forget the shades of gray surrounding every interaction. She thought it appropriate to contact the adoption agency about what I had discovered as it was public news (just public news I came across months later), but yes, agreed we don't say anything in our letters since Baby Z's mom has been holding strong to her desire to keep us at arms length.
I didn't contact the agency as the initial shock and grief wore off quickly and S and I decided that this is news we share with Baby Z because it may help to explain why she may never want contact with him, or at the very least it's a tiny piece of a bigger puzzle. I still got the itch to try to do some more digging though, because (let's face it), I'm nosy until I find an answer. Thankfully, I found what looks like positive news.
I probably won't stop the searching every now and then, gathering the data for Baby Z in the future, but I sure will be ready for anything when I do it.