We recently bought a 100 year old house that stood uninhabited for 18 months prior to our move. Everything had been properly winterized for the cold winter months (Temperatures can reach -40 C) and the water was turned on and seemed to be working properly for about 3 weeks. Then we ran into some issues. The house is two storeys with two full baths on each level stacked on top of one another. The first floor toilet had a minor leak that seemed to correct itself within a day and hasn't been an issue since. The second story toilet began to leak about a week later due to the tank overfilling and leaking through the flush handle. We remedied this problem quite easily and the toilet hasn't leaked since. Now here comes the real issue.
Shortly after the second toilet leak, we woke up early one morning to discover water covering the floor of the first floor bathroom. It was coming from the ceiling at a steady drip rate, but nothing alarming. We shut the water off to the entire house and the dripping stopped after about 12 hours. During the following week we ended up peeling back the plastered layer on the ceiling to discover that it is made of cement board (??) and the water seems to just seep through when the water is on. We can access the shower pipes on the second floor through a removable panel in the wall, but there is no water to be seen. I guess I'm just trying to see if anyone has had experience with a similar issue, any suggestions as to how it can be diagnosed/fixed, and whether or not I'm going to have to take a second mortgage out to fix the issue.
I should mention that we did call a plumber the same day the ceiling leak was discovered. He set up an appointment, but then called back and told us that if the leak was related to a burst pipe, the amount of water leaking would be much more substantial and it wouldn't continue to leak after the water was turned off (which it did initially for about 12 hours). He informed us it was probably water that had pooled underneath the upstairs toilet and was now seeping through the cement board. We now know that the leak will start and stop in relation to the water being turned on and off, but I think the issue is that the leak isn't a dramatic one when the water is left on. It just kind of... drips through. Please help!