I had exactly same signs of behavior with my 2005 FEH when air cond lost freon and same behavior of technician who could not find leak and recharged system with DYE and could not see leak again. Charged me 350 CAD and said come back if problem repeats. After two weeks system empty or partly again. I inspected system and found large trace of Green fluorescent liquid in area where condenser pipes connected. Booked appointment for next Monday. My suggestion that technicians saw the leak but they want us to come again just to get more money. If I could find that large leak by naked eye why they could not see it with leaks finding equipment even when system was charged under pressure and filled with DYE. How they could not find leak from damaged condenser in your case? Answer is one: they are no our friends.