Here's amazing video that was eye opener for me.
Apparently, there is a drain valve at the bottom of the hybrid battery case in the car body. On this vehicle, it was plugged because of all kinds of debris dropped there. Food, coins, what not. Amazing. As the result, battery had water damage.

Interesting. This was a RX400h. I wonder if any design changes were made to prevent this on our generation RX450h.

I doubt. Best will be to get underneath 450H and see if same valve is there. It makes sense to have it.
Best practice - don't junk inside your car. And vac it routinely. Vents are right there, close to floor.

hmmm..isn't there a vent cover on there?