Walls have ears and so do bars

Aberforth isn't a father and he failed being a brother but unlike Albus, who put this child in this situation, he wouldn't fail the baby in his arms. (Or Aberforth hears McGonagall ranting at the Three Broomsticks about having to leave Harry with those horrible muggles and decides to "kidnap" the Boy Who Lived so he won't have the same fate as his poor sister.)