Coolant leaks are common on many cars. Sometimes lost coolant just seems to disappear. The system is closed, so this shouldn't be happening, and it has to be going somewhere.
+1 for rickj - check the rad cap for gunk around the rubber seal, and make sure the springy part moves back and forth okay. If you have any doubts at all, pick up a new cap because they are cheap. (Change it only when the system is cool.)
Other possible causes, cheap to expensive:
1. There could be a leak at a connecting hose on the radiator or engine, or the thermostat housing or o-ring is leaking. A hose leak is more dangerous because it could suddenly become a big leak, while on the road or even when you are looking at the engine. Be careful looking around while the system is pressurized. Use safety goggles at least. If you have a face shield, use that. Wear gloves. If the system is hot, don't go poking hoses that might be brittle. And keep your hands and tools away from the fans if the engine is running - they could suddenly come on.
2. The radiator could have a leak out of the top or bottom plastic tank. These are the large black parts attached to the aluminum core - the top tank has the rad cap, connection to the reservoir and the top hose connection; the bottom has the bottom hose connection. There is a large gasket that seals each to the aluminum part of the tank and these can leak. More rarely the tanks could develop cracks, but you would lose coolant much more quickly and a pressure test should have picked this up. I've seen a leaky gasket three separate times in my '01 Forester S over its 150K miles - two leaks in the top tank, one in the bottom. If you have been driving for some time (e.g. several tankfuls) but not getting overheating, it is more likely to be the top tank. The system may only leak coolant when the engine is warmed up and the system is at pressure. A small leak would allow coolant to squirt out when the system is warm, and it might be visible for only a short while. It would be lost on the road and you may not smell it or see it. To see an active leak, you could top off the reservoir and radiator (when the system is cooled off), get the engine warm, then look under the hood while the engine is still running. Leaks at the top tank are easy to see; leaks at the bottom less so. (Same precautions as above note.) [As a side note, if you are mechanically inclined, removing the radiator is not that difficult nor time-consuming, and you could have a rad shop test/repair it for about $100 or less. A new rad is about $120-$300. If the coolant was just replaced you can save it and re-use it. A dealer could charge two hours' labor (i.e. $150 or more) on top of parts, and they would need your car for at least a day, so it is worth your while to pull the rad yourself. If you do this, change out the thermostat too with a Subaru one (about $20). Don't overtighten the connecting hoses: The tanks and thermostat housing are plastic.]
3. There could be a pinhole leak in the middle of the tank. If the shop had pressure-tested the system, they should have seen it. If you pull the rad yourself and get it to a rad shop, they will test it.
4. The engine could be losing coolant due to a blown head gasket. When the gaskets first start to go, the leaking occurs when the car is first warming up, then the leak seals and whatever coolant was lost largely disappears. You would need to look around the engine heads for any trace of lost coolant if it is an external leak. Use a flashlight and hand mirror if looking from the top. Alternatively, the coolant could be lost internally, into the combustion chamber. You should see billows of steam out of the tailpipe, and bubbles might also be visible in the coolant reservoir at times (the exhaust gases being forced into the cooling system by the leak). A compression test should pick up an internal leak. Check other threads on this site for detection of head gasket leaks.
5. An engine head or the block could be cracked. A compression test should pick this up if the crack affects a cylinder, or a cooling system pressure test should catch it.
If you are still losing coolant and can't find the leak, try removing the bottom plastic engine skirt (usually taken off for fluid changes). Put some clean white paper all around the area under the engine when it is cold (a garage is best). Start the car and let it idle. If you are losing coolant somewhere, some drops should occasionally escape and land on the paper.
As an attempt to stop a tiny leak, you could try adding stop-leak (Subaru Coolant Conditioner). If it works, be thankful and keep watching the coolant levels.
Good luck...
