This may sound terse but where you're looking MIGHT not be where the leak is. What I mean is that if you aren't seeing any signs of water or wet pipes, start looking on 'other material' for signs of past water 'trails' or 'damage'. It's possible that the leak is coming from some other place, hence, the water is traveling some downward/downhill 'path' to where it ended up in the closet. If the bathroom is on an outside wall, ask yourself if it's POSSIBLE that there's a water leak from, say ..., the roof or even a bathroom above the one that's leaking. Get some good lighting and look for the 'signs' of past water damage/trails.