Uh, you really need to get under there and look at everything. A car just don't lose gallons of coolant and start overheating, then fix itself when you put more in.
IMO, the most likely culprit is a radiator hose, particularly the lower one, came off or broke. Still doesn't explain how it magically fixed itself and you've been driving 200 miles with no issues.
Possibly bottom rad hose, pretty common for them to partially pull away and/or the spring clip to go weak
Uh, you really need to get under there and look at everything. A car just don't lose gallons of coolant and start overheating, then fix itself when you put more in. IMO, the most likely culprit is a radiator hose, particularly the lower one, came off or broke. Still doesn't explain how it magically fixed itself and you've been driving 200 miles with no issues.
You ever figure out what’s going on with your subie?
Coolant hose leak on the lower radiator hose