I have very limited experience in the sport, but I did have a recently with a tank.
Could not find the leak, until I over pressured it. It had plit on the seem right on the bung, and was invisable to the naked eye. Even blocking the tubes on blowing into the tank did not reveal the sourse of the leak, I rigged up a bycicle pump and a bucket of water. The leak was tiny, but still a leak nevertheless.