I've read the rules about (possibly) living people so won't post any identifying details.
However, I recently got in touch with a long lost cousin for the first time in about 65 years. Now I have found (via Freebmd) that she had a brother that she never knew, born in 1946 some miles away from her own birthplace. As usual, that gives his birth registration district and the quarter of the year. I have bought a copy of the birth certificate and sent it to her. The mother's details make it indisputably ours, even though the father's forename is slightly incorrect on the certificate. They separated around that time, so it may be that she deliberately declared an error.
That has been emotional enough for my cousin, and to a lesser extent for me, too. But now we are wondering what happened to him. Freebmd and other websites show no death record of him in the same area and an enquiry to the helpful registry office has not revealed any appropriate death record in the next couple of years.
That seems to leave adoption as a possibility, or of course a simple error at the registry office but they are few and far between.
If anyone has any bright ideas they are most welcome, but in particular I am looking for advice on how to trace adoptions.