How to troubleshoot a loss of coolant?

I have a 2016 Prius that has been absolutely flawless for the 5 years I've owned it since my 2010 was Totaled.
I haven't been driving it lately since I bought a Maverick Hybrid and so I lent it to my daughter to use. I needed it back to get it e-check and change out the snow tires.

While driving it one morning after it getting checked, I notice the heater wasn't working and I thought that maybe there was trouble with the blend valve was sticking. Looking at the temp gauge on the scan gauge was at 186*F to 190*F. so why was I not getting any heat?
Remembering one other time I had a similar problem on an older car, I checked the coolant level and it was empty. I added more coolant once and then a little more to top her off. That works out to be about 1/2 a gallon. The heater was working great again and engine temps were still at 186*F.

There are no spots of coolant on the garage floor or any smells inside or around the the engine.
Seems to be loosing about 100 milliliters (1/2 cup) every 60 miles.
Oil level is normal, still clear cause I just changed it about 400 miles ago.
90,000 miles on the engine.

I do have an off and on problem with the radiator shutters. I already replaced the broken shutters but I get a warning message saying they are not moving every once in a while but shutting off the car and restarting it clears that problem. I already replace the broken shutters about a year ago.

The engine run flawlessly and still giving me around 60mpg. Acceleration is great. No warning lights or anything.

I have a OBD2 scanner.
Coolant system pressure tester,
Compression gauges, leak down gauges.
All sorts of tools to get in a lot of trouble.
I will look to see if there is any steam coming out the exhaust but if it just a leak, will it het hot enough to turn to steam so I see it?

What I don't know:
Where to look for leaks or even see it if it's internal to the heat exchanger.?
How to crank the engine to get a compression test.?
How easy to replace the heat exchanger?.
Can I bypass the heat exchanger temporarily?.
Why did this just start? From sitting around to much? .
Is this possibly just the cooling system re-leveling itself because it ran so low on coolant.?

 

That would of been 2 days ago.

I was looking for wisps of vapor coming from my exhaust pipe but the engine doesn't run long enough to see any vapor.
I felt along inside the tail pipe to see if it was wet and it was but it didn't look (clear), smell (no smell) or taste (no taste) like antifreeze.

Am I wrong in thinking this is just to heat the engine up faster so it runs more efficiently?
I may bypass it until I can afford a new unit.

 

Latest update. Cooling system has held 15psi for 20 min now.
How long should I let it go for ?

 

Sorry for the miss understanding. It’s not loosing 1/2 gal everyday. I put in 1/2 a gallon when I first saw it empty. The I’ve been adding 1/2 cup last couple of days.

I extended the pressure test. Looks like 1 psi drop in pressure in 2hours and 20 min

 

Well I went to work today about 50 miles. Finally saw the steam coming out the tailpipe and when I got home I found the coolant level had dropped about 1/2” in the tank.

so I hooked up my shop vac and got to work. It took longer to get my tools together than actually performing the bypass.

why a shop vac? Loss very little coolant. As you can see in the picture.

 

Just because I didn't fully understand what the cowl one was doing and the one underneath was pretty simple.
Maybe after doing a further investigation I may switch to that one.
I procrastinated too long about the pressure test and I had to do something that day as my daughter was getting it the next day.

Good news is 200 miles and not a drop used.