Heavy rain triggered Red Triangle and check engine light. How to proceed?

It's a big distraction to try to guess what turned the warning light on. Just find out. A dealership will have a scan tool that will tell you the trouble codes, but some mechanics may too. Or you can look for the apps that can get you that info, here:

Gen2 OBD2 app review | PriusChat

You didn't say there's a lot of stuff wrong with the car, so I'm not seeing how visiting someone with a scan tool to find out why your warning light is one would turn into a $$$$ rabbit hole.

What'll happen is you'll know why the light's on. Then we can talk about whether that'll be $$$$ or not.

 

Be careful with the free scans. There are a lot of ECUs in a Prius, including several that many generic scan tools won't even know about or show you the codes from. The trouble codes that can light the master warning triangle are found in a couple of those ECUs.

So what usually happens in a thread like this when somebody has that light is there'll be a later post where they say "ok, had it scanned, there aren't any codes", and then some posts after that where we say "nah, if the light's on there are codes, that scan tool just didn't show them", and so on.

So if you go by a place that offers free scans and then they tell you there weren't any codes, you'll know you need to visit somewhere with a better scan tool (or equip yourself with one, which the thread I linked above can help with).

 

Did you fix the common cracks in the upper corners of the hatch opening?