unable to locate a/c freon leak

Crazy problem. Very rare to do those tests and not have the problem reveal itself. I don't think there is an easy solution here and the dash removal may be the only option. You could possibly try another mechanic and let him know the tests that you have already done and see if he is any more capable.

Just bought a 98 Prizm and having same problem. The main moving part on a a/c in a car is the compressor and sometimes the seal will go bad in them. What I did to find out was I bought freon with uv dye in it and filled it up. Then wait for it to run out, then wait for night fall and shine a black light in under your hood, and bam, it was the seal in my compressor. You will not see it unless you shine the black light on it. You can get that freon dye at walmart, make sure you get the ones with the hoses. Sad to say, this is a expensive jod to fix, cheapest in my area was $850. But make sure you shop around cause I am actually taking my chevy prizm to a little hole in the wall ford place that will fix it for $470 taxes and everything, what a blessing. That's compressor change out with brand new one, flush, oil & freon.