If a leak has gone undetected for quite some time, it may be picked up after a while due to a noticeable water stain growing on the ceiling or walls. These stains occur as a leak gradually worsens and is absorbed into your drywall or wallpaper. These will not stop on their own accord and will increase in size until a qualified plumber is able to resolve the problem.
Some mould and water stains may due to a leaking shower.