I've read a lot of the replies (maybe not all) and don't want to
re-iterate what was said.They are all good suggestions. One poster
mentioned the use of refrigerant and a leak detector. That gave me the
idea of canned smoke. It is used to detect drafts. I don't know how
effective it would be at detecting a leak in a compressor. One crazy
guaranteed way of finding that leak is to submerge the whole system
into water. If not the whole system, then maybe part of the system
which wont be destroyed by water. Good to test the not so obvious.
Since the system is 20 gal, that could be a challenge.

Sounds crazy perhaps, but have they ever been tested?