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This would not really be a vacuum leak, it is an intake air leak. There is no vacuum in the air intake runner, only vacuum is after the throttle plate. However, an air intake leak would allow unmetered air to bypass the MAF. This would result in lean condition, but a small leak may not throw a code, as long as the adjustments made by the ECM keeps the O2 sensor information in the proper range. The ECM does not only adjust fuel trim; but also the ignition timing and on our engines, the variable cam timing (exhaust cam). The O2 sensors operate in a range that is considered normal. Maybe you were running on the lean end of normal?
I believe you are running the 5.6K resistor in your IAT? The air intake leak would have the same affect, it would be fooling the ECM into thinking there is less mass of air flowing through the intake (since some would be going around the MAF not through it), so it would adjust the fuel trim, timing and cam. Maybe the leak was acting similar to your resistor?
PS: Let me know which line that was! I want to test it out.
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