Fair questions about leak detection
Why pay to find it when you could just open the wall?
Because exploratory demolition is expensive and often wrong. Locating first means one opening in one place. It is the cheaper path in nearly every case, and it is much cheaper when the leak turns out to be somewhere nobody would have guessed.
Can you find a leak without any damage?
Location itself is non invasive: listening, thermal and pressure work do not break anything. Reaching the leak to repair it will require access, but a located leak means a small, deliberate opening rather than a search.
What if you cannot find it?
It happens occasionally, usually with a very small intermittent leak. What you should expect is a straight account of what was ruled out and what the next step would be, not a repair quoted on a guess.