To make matters worst, you have to realize that water doesn't flow STRAIGHT downhill. It can twist and turn and all that.
There may be some plumbing somewhere away from the damaged area whereas water is making its way to that part of the wall/ceiling. As Steve says, if you can get access to the room above, go from there. If the leak is on the first floor, go upstairs and start gently pressing the drywall (if that's what you have.) If the drywall is soft and mushy, you're on to something. Work your way left and right of the area that's damaged, doing this on EACH floor of where you find damage. When you end up in the attic, do the same left and right work.
If you have an A/C unit in a window, perhaps it's leaking into and behind it. You may have to pull it out to be sure. Let us know how you make out.