I have never seen a lineset inside a wall have a leak that wasn't caused
by a nail or screw. However, all the ones I've seen (about one a year
average) leaked out about as fast as you could put it in. As was
mentioned, if indeed that is where your leak is, it can be fairly easily
repaired, usually. If the hole is fairly small, as it sounds, since
yours takes two weeks to leak down, the hole itself can probably just be
soldered-- a lot easier and safer fire hazardwise, than installing a
coupling inside the wall. If you find that the nail is not near the
lineset, the leak is most likely not in the lineset. The only other way
to check the lineset is to isolate it and pressurize, or pull a vacuum
on it, and see if it holds. Before I would do that, I would double and
triple check the evaporator coil-- we find about 80+% of leaks there. Of
course it CAN be leaking anywhere in the refrigeration circuit. What is
the brand of unit you have? Some are more prone to leaks in varous
places than others. Good luck Larry