air con leaks

I had the air con re gassed last week and all was fine for about 3 days, now it seams that it's lost all it's gas again.

Is there a simple way to find where the leak is (nothing visable on any pipes as far as I can tell), or is it a garage job as in the workshop manual is says to use a gas detector.

Did they put in a UV tracer when they topped up your gas?

Prior to having my system recharged the garage carried out a vacuum test .Only when it passed did they proceed to recharge it .
On completion I rec'd a print out of all details/specifications of test.
It's been fine ever since (2years)

I got the print out and all was fine for a few days, which makes me think it's a very small leak which takes several days to drain the gas.

Had the same problem with my Discovery. After spending money on a regas with no success. I took it to an air con engineer who tested the whole system and found a very small leak in the pipe leading to the dryer beside the radiator. I would never have found the leak because of its location and it only showed up under ultra violet light.Used a really good Air Con engineeer if you live in the West of Scotland I can pass you his details via PM so as not to advertise.