Hi
Just to add another track to take ,whilst I think BRG is on the right track
I recal a similar thing in a terraced house with 3 layers of 'Lino' on the
bathroom floor .Turns out the family had 3 active kids who were bathed most
nights and mum used a jug to wash them ,when she left them alone water
fights started and the water ran to the edge of the flooring and ran through
the edges of the room running down the walls of the toilet below also
finding its way into the cavity by way of the exiting pipework AND
electrical cables.( Yes it was wired in the cavity common then )

HTH
CJ