I would use the process of elimination on this problem. If the water eventually stops dripping, you can rule out a water line leaking. This leaves the following potential causes: 1. shower drain leaking (bad seal), 2. shower valve may be leaking only when water is on (remove trim to view) 3. shower arm may be leaking behind wall 4. toilet seal may be leaking . I always try to eliminate these possibilities first. If no luck, may have to cut ceiling to investigate further.