Best bet is to get the ac system topped up with UV tracer.
When it leaks out, the source of the leak will be visible under UV light.
Even the smallest pinhole is enough to cause loss of refrigerant.
Had this problem under warrenty and the dealers did the above and found a pinhole defect in my condenser radiator.
They replaced it as my car was v low miles and had no outside damage.
They did tell me they have lots of busted condenser radiators getting leaks cos the bumper grills are very wide and let stones etc in which cause the prob. They charge £650-700 for this work. Bit bad considering its a design flaw letting in stones!
Dealers fitted an additional stone guard to the radiator and no problems since.
Good luck getting it fixed. But condenser radiator is my best guess as the culprit.
Let us know how it goes :)
