Just before Christmas my Air Con stopped cooling............when Hobart was experiencing its 4 hot days of the year.

I got my Air Con pressure tested (tested OK), refilled with good quality refrigerant that includes a dye and leak tested using a "sniffer" that picks up leaking refridgerant.
I was expecting to replace pipes, seals, compressors, condensers etc so a bill for $180 (including 3 month guarantee) was neglible.
2 weeks later..........same thing happens..........no Air Con gas left.
Air Con system was pressure and sniffer tested again (no leaks) with no signs of dye and refridgerant refilled again at no cost.
I am due to drop the car back in to the Air Con specialist in a few days time to get re-tested.
Has anyone had a similar problem in the past? or an idea of where the gas could be going?
Cheers
Chris