I ended up taking my car to my local mechanic, and they hooked a pressure gauge to the radiator. After adding pressure, the radiator immediately started leaking. It was leaking from the plastic mounting tabs near the top of the radiator, if I'm saying this correctly?
I was hoping it it was just a loose line, or a busted hose. They explained that it's not possible to just replace the plastic mounting tabs, but that the entire radiator will need to be replaced.
There you have it, a leaky radiator.
Solution: Replace radiator.
2006 GTO, Cyclone Grey
Mods: Magnaflow Exhaust