You live in a semi-detached or terraced house so I assume there are other similar properties nearby. Have you asked other neighbours if they have the same problem with their neighbours?
I can only think that the old couple either used a commode,
as Prudie suggests, or that they had carpet and curtains in the bathroom which would have minimised the 'echo' effect of an empty room.
Just as well the bathroom doesn't adjoin your sitting room or kitchen. At least you're not troubled for most of the time.