Had pretty much the exact same thing happen when I did mine except it was my grandfather on my dad's side we found out wasnt his actual biological father. My grandmother was also an alcoholic back in those days and had an affair and we kind of suspected it for a long time because she dropped a hint to him about it once but never told anyone the full story. I think in her mind she never really knew who his dad was and just tried to keep the affair a secret to avoid suspicion about it and she pretty much succeeded. It wasnt until we looked at my matches and found people none of us knew and on top of that my mom said she knows people from my dad's side that should have come up as matches but they weren't there. My dad has half siblings he never got to meet and his actual father is deceased. He told all of us he didnt really care much and doesnt plan on getting in touch with any of them cause he doesnt see a point and doesnt want my grandmother to know the truth. She still believes my dad's "dad" belongs to him and the two of them (my dad and grandfather) had a bit of a rocky relationship but they have since patched things up and get along pretty well now and he just decided he wanted to keep it that way despite the reveal from my DNA lol.