Had a leak on a York one time that no one could find. Went on for a couple of months and no one could find it. Then one day another guy was having a go at it with out much luck. He laid his leak detector down on the top of the condenser, and the probe was hanging down on the side of the barrel. The detector went wild. The leak was a small hair line crack in the shell about halfway down. Wasn't near any welds or fittings.
Good point. Sometimes we get so used to finding leaks at fittings, connections, flanges,... etc., that we forget other possibilties like this. We found one once that was a casting flaw in the suction cover of Trane PCV. And it had been in (almost) continous service for several years, with no complaint. The old belt-drive purge was keeping up with it, and this was back in the days when some sites had that annual Spring rite of "time to dump another drum in her".