Freon leaking

My air conditioner has lost it's Freon but the mechanics can't find where it is leaking from, they have filled the system 3 times with dye and each time it is empty after a few weeks but no trace of the dye anywhere. They have had this car in 4 x's and can't find the leak after placing dye in the system and cleaning the engine the Freon leaks out but they can't find the leak. Where could the Freon be leaking from? They replaced the shredder value once.