And what were both dogs doing with themselves if a stranger was skulking around outside, and why would a non stranger be listening in.? There'd be little to hear between wall density and room size.
This is just it,because it was a farm with workers in and out day and evening,it would be something that the dogs would have been used to. As for the back,wasn't there a back door with a key left in it,only lockable before everyone retired to bed ? It may have been easy to listen-in if it was easy enough for Jeremy to just nip out with his rifle. Neville too was going back out again after his supper,so that part of the house wouldn't have been barred and shuttered.
The place would have been familiar to anyone who knew it.