I'll post the pics later, but the wife's 2012 Traverse was loosing refrigerant over the course of a few months, and for the life of me I couldn't figure out where. Even took it to a shop to have it tested, but they couldn't find the problem either.
A few months back I decided to fill the system with more AC dye and last week I found the leak. Right behind the rear cylinder bank are two hard lines that were rubbing against each other. The low pressure AC line was rubbing against the crimped portion of an adjacent P/S line and there was some significant wear on the AC line. It had developed a very minute pinhole leak. I could see the slightest trace amount of dye with a black light.
Immediate solution was to clean it up and cover the entire spot with a generous amount of JB Weld. I let it cure for a day and then tested and refilled with refrigerant. Surprisingly the JB Weld is holding and no more leak. If it holds for the rest of the season I'll order and install a new line this winter.
If you have an older GM Lambda platform car with the 3.6L V6, then I would strongly suggest you take a look back there and bend the two lines an inch or two apart so they don't have a chance to touch.