Where's the mechanical hound after Montag kills Beatty in Fahrenheit 451?
I assume that you are asking about where the hound is just before. Once Montag kills Beatty, we know just exactly where the hound is -- it is chasing Montag. It is there pretty much as soon as he kills Beatty. Then it pounces on him and gets him in the leg with the needle before he incinerates it. So you know where it is then. But that seems a little obvious so I wonder if you are asking where it is before that.
But where is it before that? Faber tells Montag that the hound is somewhere in the neighborhood of Montag's house. We do not know why it is there. Maybe it always comes along when the firemen are called out. Or maybe Beatty had it come along as a special thing this time because he thought it would be needed. We don't know, but we do know that the hound was (or at least Faber said it was) in the neighborhood even before Montag killed Beatty.