Air con not cold--- new comp 12mths ago ----.

If freon is depleting, there must be a leak somewhere and if it takes some time for it to be reduced, the leak is minimal and won't be easy to detect.
If the pros you are refering to can't detect the leak, it might mean he is not as profesional as claimed.
Adding a tester dye to the system and running it for a while would allow you to check for leaks with a ultra violet light.