We don’t like opening walls, but it’s the only way we can be sure we know where the leak is coming from.
Some home inspectors have a tool they can use to detect moisture inside a wall. Unfortunately tools like that can only tell that water exists, not where the source of the leak is located. There could be water in the walls that is flowing from somewhere else in your home – it is possible the water is running along a pipe for a bit before it drips.