Thanks for the reponses.

The leak stops at the bottom of the skimmer opening.

>I'll always dose the pool with a good amount of chlorine (5+ppm) and turn the pump off for a few days to let the pool stabilize at the level of the leak.<

We'll try that.

American Leak Detection is a franchise, so the quality of service probably depends on the franchisee and its employees. When the technician first came out, he spent 4-5 hours and couldn't find where the leak was. He had to come back with his boss (owner?) the following day and the boss found it.