I do agree with wpbharry somewhat. Most all intakes in our area leak before 50k, long before the fluid is flushed. In fact, if the vehicle is almost out of warranty we look real close for beginning signs, which you can usually find.
What causes the leaks is that the engine block is iron but the heads and intake are aluminum. The alum. expands and contracts much more than the iron, which puts alot of stress on the plastic\rubber gasket. When it gets real cold outside (10 F or less) the aluminum contracts so much that the end bolts loosen up. This makes the gaskets lose their seal. Most all of the good leakers will have the outside bolts only finger tight.
GM's fix for these leaks was originally putting blue threadlocker on the bolts when changing gaskets, now they've updated to new bolts with better threadlocker and higher torques.
Beware, when you see coolant leaking on the outside, there is a good chance just as much is leaking into the lifter valley and contaminating the oil. If yours has been leaking awhile, take off the oil cap and look for a brownish sludge. The whole inside of the engine is like this and your bearings are almost gone. I do alot of long blocks because of this.