The problem is that these AC mechanics look for leaks in the usual places, no one wants to go through the trouble of doing an extensive test for leaks, because it is very time consuming and complicated. It is easier for them to convince the customer to top up the gas, they make some money and the customer gets a working AC for the next few months.
If there is no leakage in AC lines, attachments, then the leakage is probably in the evaporator or condensor core.
Visual inspection of both should be performed to look for refrigerant leak. If nothing is found in visual inspection. An electronic leak detector device can locate small leaks , but I doubt anyone uses it in Pakistan. The other way is to remove these components and use a vacuum pump to pull a vacuum and then check the gauges for variation in pressure.
Hope that helps.
