"Check Hybrid System" every few months

The car's got an interesting history. With the timing you mention, I have to wonder if it was cleaned up from hurricane Harvey & migrated eastward...

Anyway, it's tough to tell without getting the data dump from the car. You can tool up to do this yourself, and it can be surprisingly cheap and easy. If you've already got an android phone or tablet, you can get an app and interface that lets you interrogate the car.

One thing I've heard a lot about the prius c in particular- the car is sensitive to battery temperature; lower is better. The high voltage battery under the back seat breathes air through a fan and some ductwork on the sides of the heel boards under the seat edge. Check the ducts for obstructions, maybe vacuum them out if pet hair gets in the car.

 

welcome!

anyone cleaned the battery cooling fan yet?

 

So was it a clean title? Or salvaged? That would explain alot...

As for strange substance they cleaned... If it's not flood damage and they're sure it's not battery acid, it most likely something that's been corroded by a battery acid leak and it's hard to see battery acid sometimes. Last week I sprayed down a hybrid battery chassis with vinegar and then some green cleaner and even though it looked entirely cleaned, the next morning, thanks to super wet weather, the vinegar and cleaner never dried and most of the whole chassis was white with all the battery acid that got oxidized. I say all this because perhaps they didn't do a very thorough job cleaning away all the battery acid and "strange substance" and it led to rapid corrosion soon as the electricity started flowing and now the voltage sensors they didn't clean now need to be cleaned. There's lots of other things, but that'd be my first suspicion....

 

that can be a major issue

 

Since you've got a new battery, I doubt it's got any issues...

I can't guarantee the dog is the problem, but I'd wager a few dollars to say that the dog hair and lack of vacuuming is clogging the screen on that cooling fan. Which if clogged, you'll get a message to check the hybrid system. The screen is very easy to clean and you'll be surprised at how quickly it can become clogged. I'll link to a video...

 

Did you pay for a CarFax report?

Did someone with more automotive knowledge/expertise check out the car prior to purchase?

You purchased the vehicle from an independent lot, NOT a Toyota dealer?

Who is performing this "warranty" work? The independent used car lot or the Toyota dealer?

What battery was replaced? 12V battery or the Hybrid Battery? If it truly was the Hybrid Battery, is there physical proof that they replaced it with the an OEM Hybrid from Toyota? Where is your invoice showing this to be the case?