I meant it would be easier to detect if you were having wetness upstairs as well. Then you would know the leak was up there, but the damage would be more significant. Luckily your upstairs is dry, so the leak isn't up there. If the house is completely level, it shouldn't be running down hill. Where the bulk of your wetness is, is more than likely exactly where your leak is. A wall, or walls will have to come down, regardless. I'd just as soon knock it out myself, than let the plumber do it and charge you even more for it.