In www.tomyang.net/cars/ferrari490.htm, Tom writes about mysteriously losing coolant in his Honda. A couple of years ago in our Acura, we started to lose coolant mysteriously, too. There was no sign of leaking.
As it turned out, the head was cracked and needed to be replaced. The cracks were probably the result of overheating the car and cold and lonely winter midnight when a rock hit and punctured the radiator and I had to keep driving the car for 15-20 miles without coolant on a deserted back-country highway until I was anywhere near civilization with phones to call a two truck.
About 2 years later, coolant started to disappear mysteriously. Over the course of 4-6 months, the amount of coolant disappearing increased slowly. Running the car with the coolant lid open showed no signs of cylinder head damage (no bubbles or foam). However, when the warm engine was stopped, the coolant in the expansion container started to disappear. After 2-3 cycles, the expansion container was empty. My guess is that a vaccum formed in some cylinders that sucked the coolant back in the combustion chambers.
Then one day, the engine overheated in normal traffic due to a loss of coolant and the crack in the head was discovered.