My new 240k '99 EK with the D16Y7 seems to have a phantom coolant leak.
I've been having to top off the coolant overflow reservoir twice in the past 2 days. The first time, the car almost overheated due to low coolant and the awful heat lately. I topped it off and it fixed it.
However, I just checked it and it was low again after topping it off a second time. The radiator is also full.
I have zero misfires, the engine runs beautifully, no discernable leaks, no smoke from the exhaust, and the oil isn't contaminated.
The coolant has drained almost a quarter of the overflow tank again after a 2 minute drive.
As far as I know the coolant only had to be topped off once in the past year of the previous owner's tenure.
Any ideas as to what I'm experiencing?

 

If you pull your spark plugs and shine a light down into the cylinders, if any of them are pretty clean and no carbon buildup, your car is burning the coolant and it's a bad head gasket. If you've overheated, there is a decent chance the head is no longer flat and will cause head gaskets to fail.

One mystery area coolant can vanish and not be immediately noticeable is the heater core. It will leak it into your cabin floor under the carpet. Poor heat from the heater core is typically due to an air pocket being in the core. No heat transfer at that point. Just topping the radiator doesn't purge the air from the system, you have to burp the car to do it properly. Also a bad radiator cap can cause overheating issues as the system cannot pressurize which causes the coolant to boil prematurely.

Check your cabin carpets and see if they are wet.

 

I'm going to check that tomorrow, and see if I can figure out how to diagnose a leaky cap
The car didn't quite overheat though. Definitely above ideal temps, but there wasn't smoke or anything else unusual anywhere in the engine bay. I noticed the temp guage had climbed about 3/4 of the way up from C after idling with the fans running, on a 95° day with 80% humidity. I shut it off and let it cool for a few minutes, then topped off the coolant and drove 5 minutes home with no issues whatsoever.
I don't believe it would be a bad head gasket. The car runs and idles perfect, I haven't seen any power decrease at all, my mpg is virtually unchanged, there's zero smoke out of the exhaust (if it was burning that much coolant I'd be followed by tons of white smoke), and there aren't any air bubbles in the radiator or reservoir signalling an air leak. Not to mention the oil is completely uncontaminated. I've checked it dozens of times and haven't seen anything even remotely resembling the forbidden milkshake of doom, and I changed the valve cover gasket a few days before this started happening and there wasn't any coolant pooled or mixing with oil in the head. I also haven't seen anything leaking externally onto the ground.
So far I'm inclined to think I'm either an idiot who doesn't understand fully how the cooling system works, I somehow jacked something up doing a simple maintenance job, or the coolant was so low before that I'm basically refilling the entire system?
That could be plausible because the previous old coolant turned green and the new type 2 blue juice I added made it darker in the radiator and reservoir. Clearly it's going somewhere in the system...
I honestly don't know. The car doesn't have a problem with overheating, and it never once did it for the previous owner. Even with this phantom 'leak' the temp stays exactly where it should.

 

Look at the base of the timing cover, if it's wet there, the water pump is weeping and needs to be replaced.