GM-GMC: Coolant Loss, gmc yukon xl, coolant loss

Question
I have a 2001 GMC Yukon XL 1500 5.3L Automatic transmission 223,000 miles.  In the last couple of months I have had to add coolant to the overflow tank about once a month.  I have checked for external leak and can find none.  I check both the engine oil and the transmission fuild at each gas stop for coolant and everything looks normal.  Engine runs normal and temp gage normal. No smoke that I can see in the exhaust.  Your thoughts are welcome.

Answer
There is one place that is fairly prone to leaks but is rarely seen and that is the water pump gasket (between the pump and block).  Easily replaced and you can get them seperate from the pump.  Usually they leak very slowly and the coolant evaporates so it can't be seen.  You may be able to see them with a small mirror and a good light.  Or, you will be able to tell if they were leaking by the old gaskets.
Hope that helps.