These leaks can
usually be relatively simple to locate when done properly. Although there are some that can be a real nightmare, it is unlikely, but even an odd roof leak from the other side of the house can be the culprit sometimes. Water has a funny way of finding its way down sometimes. Most of the things people have mentioned here are easy simple things that any plumber should have looked at (not that all plumbers have their chit together), but there are a number of other things that should probably be inspected. Also a big concern after finding this leak is making sure to get these areas properly dried out to ensure that you are not harboring those nasty mold guys.
A moisture gauge, camera, knowledge of how plumbing works, a little experience in leak detection, and a bit of luck, Hmmm.... that sounds like most kayak fisherman
.
I sent you a pm with my contact information.