Water is a nightmare to deal with! It travels or soaks through or whatever and can be coming from somewhere other than where it's causing the problem. That being said it shouldn't be too hard to track down. All water will stink if it's laying for any length of time but with dyes and simple tests you can find where it's coming from.
I was in the building trade before MS and know how much of a royal pain it can be. Fingers crossed it gets sorted for you soon.