2019 Prius Slow Coolant Leak

Sounds like just the ac and fans were running at first then the rattle on engine startup.

But you are losing a lot of coolant in one day. If it was coming out at the exhaust heat exchanger I would expect some steam soon after the exhaust heated up.

With that much coolant loss the combustion leak detector test might be an easy test without having to remove the windshield wiper assembly to access plugs.



Any shop could do that test as well.

If it is negative, you could do the exhaust heat bypass or move straight to a borescope test with the specific setup as described in item 2 of post 16 and as demonstrated in the videos.