Mysterious Leak At Bottom Of Interior Bathroom Door

We recently purchased a 1900s mid terrace stone built cottage which we are renovating for rental. The main house was a 2 up 2 down which has since been extended with a flat roof extension which contains the kitchen, the back entrance and the bathroom so the bathroom is at the very rear of the house. Whilst removing floor tiles from the back entrance we noticed a damp area at the base of the door frame into the bathroom. This did not seem to dry. The dampness was picked up by the plumbers I have just had in to install a new central heating system. They did a mini dig on the area and uncovered a small white pipe which we all believed was the problem. This was last Friday. I was at the house today and we had some heavy showers last night and this morning and the area looked a bit more wet than it did yesterday. I then proceeded to remove the skirting board which runs along the same wall of the leak because it looked a bit rotton anyway and behind that was all wet aswell. Do you think that the weather could be a factor and not the uncovered white pipe.