It could just be that your rad is leaking out of the lower tank. Can you smell coolant while you are driving?
I had a similar thing happen with my '01 a few years ago. The car was overheating and loosing coolant. It turned out to be the lower rad tank gasket. I had the dealer figure this out, and it cost about $400. A year ago I had the head gaskets done. Last month I noticed I was losing coolant from the top tank gasket (so it never lost enough to lead to overheating). It is hard to see an external coolant leak from the bottom tank. It leaks only when the system is pressurized, and then it is usually a trickle. The coolant splashes out onto the street and dries up if it hits the engine.
Try pulling out the radiator. You can do this yourself fairly easily. For my car it is: Drain the coolant (bottom drain will get a lot of it, easier if you put a short hose onto it). Remove the coolant reservoir. Remove the drivebelt covers from the engine. Disconnect the lower rad hose and catch the remaining coolant. Disconnect the top hose. Disconnect the fan electrical connectors. Remove the mounting brackets. Lift the rad+fans out. Take off the fans/shrouds.
Take it to a rad shop (the kind that does work for other dealers/shops) and have them pressure-test it and repair if necessary (probably cost ~$100).
I just did this last month (Nov 09). The time to remove the rad was less than an hour. A local rad shop repaired my top tank in a day for $95 (with a replacement tank+gasket, the right way to do it). I replaced the rad in less than 15 minutes, filled it up with coolant, and it has been fine since. I also replaced the thermostat while I was at it (a $20 part). The coolant looked good when I removed it, so I kept it all and put it back in, then topped off what I had lost. (If you do this yourself you should put in the Subaru coolant conditioner.)
2001 Subaru Forester S, M/T, 2.5L N/A, 165K miles