Howard Beale wrote:
What's the industry best practice for solving these leaks? Is the
supply line itself a culprit or is it typically just the solder
joints at the expansion coil or the coil itself? Is it a
soap-and-water mission over all the joints, or is there some
high-tech tool to check for pressure losses?

The best practice for leak detection is to ALWAYS ask the homeowner if
they would like the leak found. "Gas-n-Go' does a disservice to the
homeowner, the industry, and the environment. Most shops have an
electronic leak detector such as this one available:

http://www.professionalequipment.com...qx/default.htm

By the way - my experience is that most leaks are from service valves,
either because the valve core is loose or because the core was damaged
by improper installation. A good set of valve caps can solve that
problem for a few bucks. The tech should check there first.