Well as everyone knows, coolant doesn't just run out (like a/c gas)... so my lack of coolant was due to a slow leak.
How do I know all this? Because it turned into a big fcuking leak today!! Drained probably half the radiator (down to where the hole was) in < 30 seconds.
Fortunately it was while I was parking the car outside work, so I didn't get stuck anywhere ... mobile radiator repairers and $380 later, I have a new radiator + oil cooler, apparently in that rad the oil cooler is mounted inside and it's more economical to replace the whole lot than transfer the cooler.... so I have a new one of them too. Given that the radiator is somewhere in the order of $250, I'm not too upset about the price given they did it on the spot within about 3 hours of my first phonecall.
30 minute crawl home tonight, and the temp needle didn't move a single millimeter off normal operating temp