low coolant, no leaks

How would you tell the difference between antifreeze and the water created from the catalytic converter

 

Similarly if the head is warped - the leak comes by way of steam bypassing head gasket) that funnels through the exhaust system. Just read that recently.
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Toyota coolant is bright pink. When it leaks into the exhaust the liquid evaporates and leaves behind pink crystal residue and stains.

 

That would be a long way to go with a borescope, since the heat exchanger is located between the drivers and passenger seat in that center hump. Next time your under the car, it's right behind the CAT - you'll see the inlet and outlet coolant hoses.

 

that's how my mechanic diagnosed my leaky heat exchanger. not sure if he used an extra-long borescope probe, or just disconnected the cat to take a look. either way the leak was obvious visually.

 

I'm sorry, but your mechanic may be "pulling your leg". I don't know how anyone can push a scope through your CAT or your muffler to "eyeball" the heat exchanger.
It can be done, but would require some cutting and welding; which would be free and on his "dime"; if the customer declines the repair. I know I would be pretty P.O.ed if someone cut my exhaust system in half and said pay me 2K or get it out of here.

 

How is the coolant level behaving since the dealer repair? Some 2020 Primes have the same problem. Very common.