Check the lines coming off the hard line going around the block in the driver's side. There's a small line that routes up to the throttle body and ISC valve, then returns to a nipple on the lower intake manifold (between the #1 & #2 runners I think).
These lines can leak making it very hard to see and the nipple in the lower intake has to seal against the head. The coolant can leak down to the head and follow the HG "groove" to the back of the motor...foward air flow helps it do that too