I think I may have found the problem. If I did, it was an incredibly easy fix, but I'll also kick myself for making such a dumb mistake. Hopefully it truly is this simple though.
I was looking at my dad's Saab 92-x Aero, as he thinks he has a cooling system leak as well. I was looking at his radiator to make sure the cap was the same as mine so I could use the same adapter on it when I realized his caps were installed reverse of what mine are. I then proceed to Google search "STi engine bay" and lo and behold my caps are on bass-ackwards. The 1.1bar cap is on my radiator and the 1.3bar cap is on my upper tank. Best part of that? They've been like that since I flushed the coolant on July 6th, 2011 (checked my service records to see when I did it).
So follow me on this one: With the lower pressure cap on the radiator, it would vent coolant out to the overflow sooner. The boiling point of the coolant in the radiator would also effectively be lowered, as I understand it, because higher pressure raises the boiling point. Assuming that coolant was venting too quickly from the radiator and its boiling point was reduced, I could have boiled coolant in the system which would have re-pressurized it. This would explain why the system was still pressurized when I removed the caps after it started overheating. It would also explain why the car was perfectly fine after I refilled the radiator and drove it home like I normally would drive it, and shows no other sign of head/gasket failure or damage.
Again, let me reiterate that I do not drive my car hard on a daily basis, so the coolant probably never got hot enough to cause this issue before, even though the caps have been switched for over a year now. If you guys knew how I normally drove my car you'd take it away from me and tell me I don't deserve it because I'm forcing it to live a sad, miserable life. I drive my 93 Impreza L 100x harder than I drive my STi, mostly because you have to drive it hard to get it to go anywhere, but also because I'm not worried about anything happening to it.
Anyhow, I'm still going to look into some of the easy things to check that you all have suggested and take the car out to try to replicate the issue, but I think I may have already found it.
What do you all think?