I ahve a 2003 Mercury Sable, 90k miles; I’m losing coolant at the rate of a about a quart every two weeks, but I can’t dtermine where the leak is…no viable leaks…no puddles on the ground after letting it idle in the driveway for 15 minutes.
If it isn’t leaking to the outside world it has to be leaking into the cylinders and being sent out the exhaust as vapor.
First get the system pressure tested. If that discloses nothing, have pressure leakdown tests done on the cylinders.
Look to see if fluid is leaking out your tailpipe. If it is you have coolant in your engine coming out your exhaust manifold.
Does your oil look normal or does it look like a milkshake?
Oil looks normal
Ok then you likely don’t have coolant in the oil, but now you should have someone with a snifter check to see if there is any exhaust in the coolant. (Snifter is a tool not his nose. 
I know this is a bit old, but in case anyone else has a similar problem:
My 1994 Mercury Sable 3.8L V6 had a problem where the coolant was disappearing with no trace. I had it tested multiple times and was told to just add more coolant. The rate of disappearance increased more and more as time went on over a period of a couple months.
It turned out that I needed to replace the head gasket. My check engine light has been going on and off since 2004 because my ECO solenoid is in need of replacement. I don’t know if it is related or not. But if you’re in a similar position, knowledge is power.