Smoke test an AC system? That only works if there is a gross leak, not a minor leak like you describe. AC systems leaks are so small, because of the high presures involved, that a dye is the only good test. And lots of dye!

You could be leaking from a number of areas and probably on the high pressure side of the circuit. An evaporator leak is the hardest to find only because it cannot be inspected, I would look first at the most obvious places in the engine bay:

Coolant hose going from the compressor to the condenser along the bottom of the car.
Accumulator/dryer bottle can crack or get porous.
If you leave the AC switched off for long periods of time, the compresor seals can dry out and start leaking.

Find a new AC shop. One that deals mainly in auto AC, not an engine repair facility. Loosing 6 ounces a year is not much, most of us loose 6 ounces a month.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)

Where are they checking for leaks? If you're losing freon, you have a leak. It doesn't just disappear.

I don't know what a smoke test is, but usually you put freon with die in it into the system. The dye is visible with a black light, so you fill it up, run the AC for a while and then look for the die at likely leak locations.

Are they checking the evaporator? Search the forum and you will find high numbers of posts about evaporator failure. Hopefully, it's just a leak in a connection or something.

You can get the freon with die at Wal-Mart and do it yourself. I have never had to do it, but there are probably instructions posted by someone here or over on Volvospeed.

Good luck!