If the coolant isn't leaking on the ground, the windshield isn't fogging up with a film when you use the defroster, and you don't smell that odd coolant smell around the engine or in the cabin ..... well the coolant isn't disappearing .... it must be going somewhere .... most likely into the engine past the head gasket or a crack in the head. You generally can't see small cracks in the head since they are on the underside of the head between coolant passages and cylinders. There is another remote possiblity .... the fuel heater. The fuel heater uses coolant under pressure to preheat the fuel before going into the injection pump .... if the heater is bad then coolant could be forced into the fuel and then ingested by the engine. My 1987 was slowly eating fuel at around 110,000 miles over about a year so I took it to a dealer to have the "professionals" figure out the problem. They said everything checked out fine .... so I took the car on a 2,000 mile road trip .... of course it only made it about halfway when it began overheating .... coolant was way down and I had to keep filling it up just to limp into a dealer .... who checked it out and cirmised it was a blown head gasket. They took off the head and unfortuantely for us found a very small crack on the underside of the head. We left the car with them and flew home. They ultimately found a used head and reassembled everything ... some three months later we flew back to pick up the car and take it for a mini roadtrip vacation home. That was about 15 years ago and the car has traveled almost 200,000 miles on that used head and hasn't lost a drop of coolant since. I suspect your problem is the same as mine was .... if you don't push it you can probably go for some time by just adding coolant every month or so .... but push the engine on a long trip or under heavy load and it could fail and leave you stranded. Sorry to add to your concerns about the issue .... but that's what happened to us. Good Luck.