A leak detector is part of your tool kit to locate leaks. I've found them very useful for locating leaks in evaporator coils. Many leaks will leave an oil trace.. not all of them will. It's sometimes hard to find an oil trace or get bubbles off of a leak in the middle of a 3 or 4 row deep coil.. Most of the time I find the leak with the detector, a trace of refrigerant and nitrogen. Leaks can show up or hide as conditions change, most show up with under high pressure, vibration or temperature changes.
“I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.”
― Benjamin Franklin