Here is my issue; Car runs great. When it runs, it doesnt leak. Not a drip. When you turn it off, it doesnt leak (slight drip, that causes wet spot over night) until the coolant temp goes down all the way to ambient temp (which is part of the mystery I have recently figured out). I have followed all the coolant lines that are visible from the engine bay and from underneath the car, and have found no leak. I have had the car up on jacks about 4 or 5 times, but it fails to leak whenever I am trying to find where its comming from. Seems like the exit point of the leak is along the oil pan, passanger side. As you can see traces of dried coolant on it.

Also, if i park the car, and open the reservoir tank to relieve the pressure before the car cools off the leak does not occur.

 

I had the same issue on my 4.2 c5 (2002). I had hairline cracks in the reservoir which weren't noticeable until one day I saw tiny bubbles from the hot coolant showing me the issue. If these cracks are on the bottom side of the tank, you won't really see it until you take it out completely. If you still have the original coolant tank and it has turned smoke-yellow on you, the chances are high that this your issue.

Get a new one for around 60 bucks and get it replaced. Don't try that stuff that supposedly seals leaks - waste of money in my opinion. You fix one crack or leak and a month later the next one pops up because of the age of the plastic material

 

2.8 c5 1998

I have a similar issue. I cannot see the coolant leak but it leaks sometimes. I will have to check the bottom of my reservoir tank.