It could be anywhere. Check the charge tube from intercooler to the turbo for loose clamps or damaged hoses. If it is a rear mount, check where the tube passes through the bulkhead, usually above the clutch. Sometimes there is a slice like a saw would leave there. You may need to shine a light into the tube and look for leakage outside. Also check the small lines (1/8" or so) that run from the gauge to the controller and to the turbo. The only other place is between the intercooler and the manifold. That is harder to check. The other possibility is an exhaust leak. The hard part to check is right off the engine, which requires the gas tank to be removed. That is also where they will usually be. Good luck.
If you find nothing, there are some good mechanics around. Neil at Boost it treated me well.
If it's a vacuum leak issue, the only part of the system seeing vacuum at idle is between the throttle plates on the throttle body and the engine. Has to be one of the small lines on the engine side of the throttle bodies. They feed everything from your boost gauge to the fuel pressure regulator. If your running the small white 1/8 plastic line from your boost gauge down to the rubber fitting supplied with the gauge they have been known to crack up. Happy hunting.