I have had this happen a few times now and I want to put this out there to help others.
I have a 2007 G6 with about 70k miles on it. At 55k miles I noticed the car running hot and a coolant smell coming from the engine. I noticed the coolant recovery tank was low, I added coolant and as soon as the engine was started it leaked right back out.
To make a long story short I ended up replacing the thermostat and coolant recovery tank. The tank was still leaking after this was done. The problem ended up being the coolant recovery tank cap. Unlike most cars this tank has an actual 15 psi radiator cap on it. When it goes bad the coolant will just pour out the bottom of the tank when the car is running. I've had it go bad 3 times now, about every 8 months.
A mechanic at the Pontiac dealership said he's been replacing these on an average of once a week. So if you noticing your tank is leaking/low try replacing the cap. It costs about 6$ at your local parts shop.
Just putting this out there in case anyone else has a similar problem... for over a year I had a coolant leak, the car never ran hot, I never smelled coolant or saw any on my driveway etc, but my low coolant light was constantly coming on... every time I would have an oil change, about 2 weeks later the light would come on until I topped it up again...
I took the car in at least 3 or 4 times for the same problem and every time they insisted they could not find a leak. This was to the dealership under regular warranty. On the 4th or 5th time they said they found a hose with a leak, replaced the hose, and for a couple of months I had no problem. This would be during 2009....
In January 2010.. all of a sudden the low coolant light started coming on again.. I took it back in, this time to a different dealership as my pontiac dealership had closed. They found a "crack in the radiator" and replaced the "entire radiator"... anyway... a few days later.. same problem... turns out the new radiator also has a crack in it... so they had to get another new radiator and put that in... no problems since and it has been several months...
The car only had 34,503 miles when the radiator was replaced...
Part # Replaced: 15873468 Radiator
Cost: 402.50 Labor/413.52 Parts
Paid by extended warranty company.
Complaint: low coolant warning light stays on
Mechanic notes: pressure test or leakes - radiator leaking at side tank.
Well, off to the dealer its going on Friday of next week. Coolant overflow tank was 100% empty. This morning I heard the water sloshing sound as well(2nd time). So would coolant related issues be covered on the 5/100?