Pipes don't leak: house does

When I just moved into the house I noticed some water on the floor in the storage. That's a bit disconcerting! But I had a lot on my mind at the time and I ignored it. And then, when my sister was visiting, it happened again. And now the rest of the house was pretty much OK so this time I paid attention to it. It clearly came through the hole the water pipe for the heating came through. Oh dear! I decided to notify the plumber. And he came over. He agreed with me about its provenance, so he took up the floor board above. But there was no sign of any pipe leakage! There wasn't even a joint in the pipe. Nothing. So he screwed the floorboard back and told me it wasn't the plumbing. But that means it must be the house! The good news is: it doesn't happen very often. The bad news is: a leaking house can mean all sorts of trouble. I will have to figure out where it goes wrong! I bought some waterproofing stuff a while ago for a leak into the conservatory. I haven't used it yet! Maybe it will come in handy now for the front of the house!