Coolant.

Watch the level on the reservoir for a week or two. If it continues to drop, could be failing coolant hose, radiator damage. Or head gasket failing.

How many miles on it?

 

First engine coolant change is slated for 100K miles or 10 years. First inverter coolant change is 150K miles or 15 years. Thereafter, both are 50K miles or 5 years.

 

Coolant doesn’t just vent out of your car unless there’s a leak or a serious overheat event. If there’s no leak when the engine is hot, maybe the leak is in the heater core, which means it will leak when the heater is on.

If it seems ok... proceed with care until you’re sure the problem doesn’t return. Monitor the coolant level at least daily.

As for a coolant flush, I think you should wait two weeks, to be sure you don’t have a leak.

 

Pixel recently asked

Other than a guess in terms of 'for good measure' or 'its about time' preventative maintenance, how would one 'know' (for certain) the coolant fluid would be bad, in need of replacement?


How to Test Antifreeze - Hydometer Refroctometer Test Strips

 

If you are getting vapor from the interior vents, this may point to a leak in the heater core. If this is the case, you wil also get a slimy/sticky buildup on your interior glass from the glycol.

 

That’s what I was thinking... which explains why the problem went away... it’s summertime and hot out. He isn’t using the heater.

 

If it is a heater core leak (sounds likely), wouldn't the heater loop continue to be pressurized similarly to the ICE cooling loop and therefore continue to leak? Unless of course there are two valves controlling the heater loop flow, which I doubt but don't know for sure. The only other time I've seen vent vapor such as this was under high humidity air intake from outside, which the OP does state sans any statement about humidity (location is US which could be anywhere), and initial air conditioner operation. After a while the interior air is cooled and de-humidified enough to eliminate vapor condensation from the vent.

 

What about inhaling ethylene glycol mist, If that's indeed what's happening: that can't be good. You might get drunk, for starters.

Ethylene Glycol and Propylene Glycol Toxicity: How Are People Exposed to Ethylene Glycol? | ATSDR - Environmental Medicine & Environmental Health Education - CSEM

 

Did you turn on the heater?

 

Did you have the AC on? Was the humidity high? Could have been cold AC air causing warm humid air to condense into visible vapor. Low coolant level may be a coincidence and have nothing to do with it.

 

Ok, this is completely off the wall, but am I the only one thinking your user name is MARTINI, and you joined in 2014.

Good coffee...

 

Those are my moments here also but with dark red wine.

 

Or a martini.

 

Add head gasket failure to that list. Check through the spark plug holes with a boroscope, and/or leak-down test.