Originally Posted by Guzzler
Actually, go find an A/C tech that uses a "sniffer".
Yup, we tried using one of those too - with mixed results. After going over every possible point in the refrigerant system at least 3 times we came to the conclusion that the most likely place it would be leaking would be at the compressor return line (low side - same line you would put your charge into) We also tried a device that "listens" for any kind of air leak with no luck.
A friend of mine's `02 WRX leaked it's freon out. I believe it's on his low-side maintenance port cuz directly behind it on the firewall there was a thin film of oil with a layer of dust/dirt collected.
I've got a thin layer of oil/dirt directly behind my compressor return line, problem is that's also very close to where you fill the engine oil with, so it's possible that that could be blow-back but I'd say it's refrigeration oil.